Editing Roman calendar (section)

==History==
[[File:Fasti Praenestini Massimo n1.jpg|thumb|right|250px|The remains of the ''{{lang|la|[[Fasti Praenestini]]}}'', containing the months of January, March, April, and December and a portion of February.{{sfnp|''Enc. Brit.''|1911|p=193}}]]
{{anchor|Lunar|Lunar calendar|Prehistory|Prehistoric}}

===Prehistoric calendar===
The original Roman [[calendar]] is usually believed to have been an [[observational calendar|observational]] [[lunar calendar]]{{sfnp|Mommsen & al.|1864|p=[https://archive.org/stream/historyrome00dickgoog#page/n242/mode/2up 216]}} whose months ended and began from the [[new moon]].{{sfnp|Michels|1949|pp=323–324}}{{sfnp|Grout|2023}} Because a [[lunar phase|lunar cycle]] is about 29.5 days long, such months would have varied between {{nowrap|29 and 30 days}}.<ref name=mommy/> Twelve such months would have fallen {{nowrap|10 or 11 days}} short of the [[solar year]] and, without adjustment, such a year would have quickly rotated out of alignment with the seasons<ref name=mommy/> in the manner of the [[Islamic calendar]]. Given the seasonal aspects of the calendar and its associated [[ancient Roman religion|religious]] [[ancient Roman festivals|festivals]], this was presumably avoided through some form of arbitrary curtailment or intercalation<ref name=mommy/> or through the suspension of the calendar during winter.
Against this, [[Agnes Kirsopp Lake Michels|Michels]] has argued that the early calendars used by Rome and its neighbors were more probably observational of seasonal markers in nature (the [[leafing]] of trees), animal behavior (the [[bird migration|migration]] of birds), and the [[agriculture in ancient Rome|agricultural cycle]] (the ripening of [[Agriculture in ancient Rome#Grains|grain]]) combined with observation of [[star]]s in the [[night sky]].{{sfnp|Michels|1949|p=330}} She considers that this more sensibly accounts for later legends of [[Romulus]]'s [[#Romulus|decimal year]] and the great irregularity in Italian month lengths recorded in [[Censorinus]].{{sfnp|Michels|1949|p=330}}<ref>[[Censorinus]], ''[[De Die Natali|The Natal Day]]'', [https://elfinspell.com/ClassicalTexts/Maude/Censorinus/DeDieNatale-Part2.html#topref94 Ch. XXII].</ref> Roman works on agriculture including those of [[Cato the Elder|Cato]],<ref>[[Cato the Elder|Cato]], ''[[De Agri Cultura|On Agriculture]]''.</ref> [[Varro]],<ref>[[Varro]], ''[[Res Rusticae|Farming]]''.</ref> [[Vergil]],<ref>[[Vergil]], ''[[Georgics]]''.</ref> [[Columella]],<ref>[[Columella]], ''[[De Re Rustica (Columella)|On Farming]]''.</ref> and [[Pliny the Elder|Pliny]]<ref>[[Pliny the Elder|Pliny]], ''[[Natural History (Pliny)|Natural History]]''.</ref> invariably date their practices based on suitable conditions or upon the [[rising of stars]], with only occasional supplementary mention of the [[#Republican calendar|civil calendar]] of their times{{sfnp|Michels|1949|p=330}} until the 4th or 5th century author [[Rutilius Taurus Aemilianus Palladius|Palladius]].<ref>[[Rutilius Taurus Aemilianus Palladius|Palladius]], ''[[De Re Rustica (Palladius)|On Farming]]''.</ref> [[Augury]], formal Roman [[ornithomancy]], continued to be the focus of a [[augur|prestigious dedicated priesthood]] until at least the end of the 4th century.{{sfnp|Wissowa|1896}} Although most Roman festivals in the historical record were closely tied to the [[nundinal cycle]] of the [[#Numa|later calendar]],{{sfnp|Michels|1949|p=322}} there remained several [[moveable feasts]] ({{lang|la|[[feriae conceptivae]]}}, "proclaimed festivals") like the [[Sementivae]] that were dependent on local conditions.{{sfnp|Michels|1949|p=331}} Michels suggests this was the original state of all ancient festivals, marking divisions between the seasons and occasions within them.{{sfnp|Michels|1949|p=331}}

{{anchor|Romulus|Romulan calendar|Calendar of Romulus}}

===Legendary 10-month calendar===
The Romans themselves usually described their first organized year as one with ten fixed months,{{sfn|Macrobius|loc=Book I, Ch. 12, §3}}{{sfnp|Kaster|2011|p=137}} a decimal division fitting general Roman practice.{{sfnp|Mommsen & al.|1864|p=[https://archive.org/stream/historyrome00dickgoog#page/n242/mode/2up 217]}} There were four months of "31" days{{sfn|Macrobius|loc=Book I, Ch. 12, §3}}{{mdash}}March, May, Quintilis, and October{{mdash}}called "full months" ({{lang|la|[[wikt:pleni menses#Latin|pleni menses]]}}) and six months of "30" days{{sfn|Macrobius|loc=Book I, Ch. 12, §3}}{{mdash}}April, June, Sextilis, September, November, and December{{mdash}}called "hollow months" (''{{lang|la|[[wikt:cavi menses#Latin|cavi menses]]}}'').<ref>[[Censorinus]], [[Macrobius]], and [[Gaius Julius Solinus|Solinus]], cited in {{harvp|Key|1875}}</ref><ref name=mommy>{{harvp|Mommsen & al.|1864|p=[https://archive.org/stream/historyrome00dickgoog#page/n244/mode/2up 218]}}.</ref> These "304"&nbsp;days made up exactly 38&nbsp;[[nundinal cycle]]s. The months were kept in alignment with the moon, however, by counting the [[new moon]] as the last day of the first month and simultaneously the first day of the next month.{{sfnp|Grout|2023}} The system is usually said to have left the remaining two to three months of the year as an unorganized "winter", since they were irrelevant to the farming cycle.{{sfnp|Grout|2023}} [[Macrobius]] claims the 10-month calendar was fixed and allowed to shift until the summer months were completely misplaced, at which time additional days belonging to no month were simply inserted into the calendar until it seemed things were restored to their proper place.{{sfn|Macrobius|loc=Book I, Ch. 12, §39}}{{sfnp|Kaster|2011|p=155}} [[Gaius Licinius Macer|Licinius Macer]]'s lost history apparently similarly stated that even the earliest Roman calendar employed intercalation.<ref name=censibility>[[Censorinus]], ''[[De Die Natali|The Natal Day]]'', [https://elfinspell.com/ClassicalTexts/Maude/Censorinus/DeDieNatale-Part2.html Ch. XX].</ref>{{sfn|Macrobius|loc=Book I, Ch. 13, §20}}{{sfnp|Kaster|2011|p=165}}

Later Roman writers usually credited this calendar to [[Romulus]],{{sfn|Macrobius|loc=Book I, Ch. 12, §§5 & 38}}{{sfnp|Kaster|2011|pp=137 & 155}} their [[Roman legend|legendary]] [[Kings of Rome|first king]] and [[culture hero]], although this was common with other practices and traditions whose origin had been lost to them. [[Censorinus]] considered him to have borrowed the system from [[Alba Longa]],<ref name=censibility/> his supposed birthplace. Some scholars doubt the existence of this calendar at all, as it is only attested in late Republican and Imperial sources and supported only by the misplaced names of the months from September to December.<ref name=rupparena>{{harvp|Rüpke|2011|p=[https://books.google.com/books?id=pThna2LDwDsC&pg=PA23 23]}}.</ref> [[Jörg Rüpke|Rüpke]] also finds the coincidence of the length of the supposed "Romulan" year with the length of the first ten months of the Julian calendar to indicate that it is an {{lang|la|[[a priori]]}} interpretation by late Republican writers.<ref name=rupparena/>

{| class="wikitable plainrowheaders" style="margin:1em auto;"
|+ Calendar of Romulus
|-
! scope="col" | English
! scope="col" | Latin
! scope="col" | Meaning
! scope="col" | Length in days{{sfn|Macrobius|loc=Book I, Ch. 12, §3}}{{sfnp|Kaster|2011|p=137}}
|-
| [[March]] || [[Martius (month)|Mensis Martius]] || Month of [[Mars (mythology)|Mars]]
| style="font-weight: bold;"| 31
|-
| [[April]] || [[Mensis Aprilis]] || Month of Apru ([[Aphrodite]])<ref>{{cite web | title=April | website=Dictionary.com Unabridged | publisher=Randomhouse Inc. | url=http://www.dictionary.com/browse/april | access-date=January 9, 2018}}</ref>
| 30
|-
| [[May]] || [[Mensis Maius]] || Month of [[Maia (mythology)|Maia]]<ref>{{cite web | title=May | website=Dictionary.com Unabridged | publisher=Randomhouse Inc. | url=http://www.dictionary.com/browse/may | access-date=January 9, 2018}}</ref>
| style="font-weight: bold;" | 31
|-
| [[June]] || [[Iunius (month)|Mensis Iunius]] || Month of [[Juno (mythology)|Juno]]
| 30 
|-
| [[July]] || [[Mensis Quintilis]]<br>Mensis Quinctilis{{sfnp|Blackburn & al.|1999|p=669}} || Fifth Month
| style="font-weight: bold;" | 31
|-
| [[August]] || [[Mensis Sextilis]] || Sixth Month
| 30
|-
| [[September]] || [[September (Roman month)|Mensis September]] || Seventh Month
| 30
|-
| [[October]] || [[October (Roman month)|Mensis October]] || Eighth Month
| style="font-weight: bold;" | 31
|-
| [[November]] || [[November (Roman month)|Mensis November]] || Ninth Month
| 30
|-
| [[December]] || [[December (Roman month)|Mensis December]] || Tenth Month
| 30
|- 
! scope="row" colspan="3" style="text-align: right;" | Length of the year:
! style="text-align: left;" | 304
|}

Other traditions existed alongside this one, however. [[Plutarch]]'s ''[[Parallel Lives]]'' recounts that [[Romulus]]'s calendar had been solar but adhered to the general principle that the year should last for 360&nbsp;days. Months were employed secondarily and haphazardly, with some counted as 20&nbsp;days and others as 35 or more.<ref name=gossipgirl/><ref name=worchestershire/> Plutarch records that while one tradition is that [[Numa Pompilius|Numa]] added two new months to a ten-month calendar, another version is that January and February were originally the last two months of the year and Numa just moved them to the start of the year, so that January (named after a peaceful ruler called [[Janus]]) would come before March (which was named for Mars, the god of war).<ref>{{harvp|Plutarch|loc=''Life of Numa'' section XIX}}</ref>

Rome's 8-day week, the [[nundinal cycle]], was shared with the [[Etruscans]], who used it as the schedule of royal audiences. It was presumably a part of the early calendar and was credited in [[Roman legend]] variously to [[Romulus]] and [[Servius Tullius]].

{{anchor|Numa|Calendar of Numa|Numan reform}}

===Republican calendar===
The attested calendar of the [[Roman Republic]] was quite different. It had twelve months, already including [[Ianuarius|January]] and [[Februarius|February]] during the winter.

According to Livy, it was Numa Pompilius, the second king of Rome {{nowrap|(715–673 BC),}} who divided the year into twelve lunar months (History of Rome, I.19). Fifty days, says Censorinus, were added to the calendar and a day taken from each month of thirty days to provide for the two winter months: Januarius (January) and Februarius (February), both of which had 28 days (The Natal Day, XX). This was a lunar year of 354 days but, because of the Roman superstition about even numbers, an additional day was added to January to make the calendar 355 days long. Auspiciously, each month now had an odd number of days: Martius (March), Maius (May), Quinctilis (July), and October continued to have 31; the other months, 29, except for February, which had 28 days. Considered unlucky, it was devoted to rites of purification (februa) and expiation appropriate to the last month of the year. (Although these legendary beginnings attest to the venerability of the lunar calendar of the Roman Republic, its historical origin probably was the publication of a revised calendar by the Decemviri in {{nowrap|450 BC}} as part of the [[Twelve Tables]], Rome's first code of law.)
{{sfnp|Grout|2023}}

The inequality between the lunar year of 355 days and the [[tropical year]] of 365.25 days led to a shortfall over four years of (10.25 × 4) = 41 days.<ref name=mommy/> Theoretically, 22 days were interpolated into the calendar in the second year of the four-year cycle and 23 days in the fourth.<ref name=mommy/> This produced an excess of four days over the four years in line with the normal one day excess over one year. The method of correction was to truncate February by five days and follow it with the intercalary month which thus commenced (normally) on the day after February 23 and had either 27 or 28 days. February 23 was the [[Terminalia (festival)|Terminalia]] and in a normal year it was ''{{lang|la|a.d. VII Kal. Mart.}}'' Thus the dates of the festivals of the last five days of February were preserved<ref name=rupkenuma1>{{harvp|Rüpke|2011|p=[https://books.google.com/books?id=pThna2LDwDsC&pg=PA40 40]}}</ref> on account of them being actually named and counted inclusively in days before the [[kalends]] of March; they were traditionally part of the celebration for the new year. There was occasionally a delay of one day (a {{lang|la|dies intercalaris}} being inserted between February 23 and the start of the {{lang|la|mensis intercalaris}}) for the purpose of avoiding a clash between a particular festival and a particular day of the week (see {{slink|Hebrew calendar|Rosh Hashanah postponement rules}} for another example).  The Roman superstitions concerning the numbering and order of the months seem to have arisen from [[Pythagoreans|Pythagorean]] superstitions concerning the luckiness of [[odd number]]s.<ref name=momma>{{harvp|Mommsen & al.|1864|p=[https://archive.org/stream/historyrome00dickgoog#page/n244/mode/2up 219]}}.</ref>

These Pythagorean-based changes to the Roman calendar were generally credited by the Romans to [[Numa Pompilius]],{{sfnp|Grout|2023}} [[Romulus]]'s successor and the second of [[kings of Rome|Rome's seven kings]], as were the two new months of the calendar.{{sfn|Macrobius|loc=Book I, Ch. 12, §34}}{{sfnp|Kaster|2011|p=153}}{{efn|[[Plutarch]] reports this tradition while claiming that the months had more probably predated or originated with Romulus.<ref name=gossipgirl/><ref name=worchestershire/>}} Most sources thought he had established intercalation with the rest of his calendar.{{citation needed|date=March 2017}} Although [[Livy]]'s Numa instituted a lunar calendar, the author claimed the king had instituted a 19-year system of intercalation equivalent to the [[Metonic cycle]]{{sfnp|Livy|loc=Book I, Ch. 19, §6}} centuries before its development by [[Babylonian astronomers|Babylonian]] and [[Ancient Greek astronomy|Greek astronomers]].{{efn|This equivalence was first described by [[Temple Stanyan|Stanyan]] in his history of ancient Greece.{{sfnp|Stanyan|1707|p=[https://archive.org/stream/grecianhistoryf00stangoog#page/n373/mode/2up 330]}}}} [[Plutarch]]'s account claims he ended the former chaos of the calendar by employing 12{{spaces}}months totalling 354{{spaces}}days—the length of the [[synodic month|lunar]] and [[ancient Greek calendars|Greek years]]—and a biennial intercalary month of 22{{spaces}}days called [[Mercedonius]].<ref name=gossipgirl/><ref name=worchestershire/>

According to Livy's [[Periochae]], the beginning of the consular year changed from March to 1{{spaces}}January in 153{{spaces}}BC to respond to a rebellion in Hispania.<ref>Livy, ''Periochae'', [https://www.livius.org/sources/content/livy/livy-periochae-46-50/#47.1 47.13 and 47.14]: "[47.13] In the five hundred and ninety-eighth year after the founding of the city, the consuls began to enter upon their office on 1{{spaces}}January. [47.14] The cause of this change in the date of the elections was a rebellion in Hispania."</ref> Plutarch believed Numa was responsible for placing January and February first in the calendar;<ref name=gossipgirl/><ref name=worchestershire/> [[Ovid]] states January began as the first month and February the last, with its present order owing to the [[Decemvirs]].{{sfnp|Ovid|loc=Book II}}{{sfnp|Kline|2004|loc=[https://www.poetryintranslation.com/PITBR/Latin/OvidFastiBkTwo.php#anchor_Toc69367683 Book II, Introduction]}} [[W. Warde Fowler]] believed the [[Ancient Roman religion|Roman priests]] continued to treat January and February as the last months of the calendar throughout the Republican period.{{sfnp|Fowler|1899|p=5}}

{|class="wikitable" style="margin:.5em auto;line-height:1.4"
|+ Roman Republican calendar ({{circa|700 BC}} or {{circa|450 BC}} – 46 BC)
|-
!scope="col" colspan="3" rowspan="2"| English
!scope="col" colspan="3" rowspan="2"| Latin
!scope="col" colspan="2" rowspan="2"| Meaning
!scope="col" colspan="4"| Length in days{{sfn|Macrobius}}{{sfnp|Kaster|2011}}<ref name=gossipgirl>{{harvp|Plutarch|loc=''Life of Numa'' section XVIII}}.</ref><ref name=worchestershire>{{harvp|Perrin|1914|pp=[https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Numa*.html#Romulan_year 368 ff]}}.</ref>
|-
!scope="col" style="line-height:1.25"| 1st<br /> year<br /> <small>([[Common year|cmn.]])</small>
!scope="col" style="line-height:1.25"| 2nd<br /> year<br /> <small>([[Leap year|leap]])</small>
!scope="col" style="line-height:1.25"| 3rd<br /> year<br /> <small>(cmn.)</small>
!scope="col" style="line-height:1.25"| 4th<br /> year<br /> <small>(leap)</small>
|-style="background:#CFC"
|style="text-align:right"| 1. ||colspan="2"| January
|style="text-align:right"| I. ||colspan="2"| [[Mensis Ianuarius]]
|colspan="2"| Month of [[Janus]]
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|-style="background:#CFC"
|rowspan="2" style="text-align:right"| 2. ||colspan="2"| February
|rowspan="2" style="text-align:right"| II. ||colspan="2"| [[Mensis Februarius]]
|colspan="2"| Month of the [[Lupercalia|Februa]]
|style="text-align:center"| 28
|style="text-align:center"| 23
|style="text-align:center"| 28
|style="text-align:center"| 23
|-style="background:#CFC"
|style="border-top:hidden;border-bottom:hidden;width:1em"| &nbsp; || Intercalary Month
|style="border-top:hidden;border-bottom:hidden;width:1em"| &nbsp; || Intercalaris Mensis ([[Mercedonius]])
|style="border-top:hidden;border-bottom:hidden;width:1em"| &nbsp; || Month of Wages
|style="border-top:hidden;text-align:center"| &nbsp;
|style="text-align:center"|'''27'''
|style="border-top:hidden;text-align:center"| &nbsp;
|style="text-align:center"|'''28'''
|-
|style="text-align:right"| 3. ||colspan="2"| March
|style="text-align:right"| III. ||colspan="2"| [[Martius (month)|Mensis Martius]]
|colspan="2"| Month of [[Mars (mythology)|Mars]]
|style="text-align:center"| 31
|style="text-align:center"| 31
|style="text-align:center"| 31
|style="text-align:center"| 31
|-
|style="text-align:right"| 4. ||colspan="2"| April
|style="text-align:right"| IV. ||colspan="2"| [[Mensis Aprilis]]
|colspan="2"| Month of [[Aphrodite]] – from which the [[Etruscan language|Etruscan]] Apru might have been derived
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|-
|style="text-align:right"| 5. ||colspan="2"| May
|style="text-align:right"| V. ||colspan="2"| [[Mensis Maius]]
|colspan="2"| Month of [[Maia]]
|style="text-align:center"| 31
|style="text-align:center"| 31
|style="text-align:center"| 31
|style="text-align:center"| 31
|-
|style="text-align:right"| 6. ||colspan="2"| June
|style="text-align:right"| VI. ||colspan="2"| [[Iunius (month)|Mensis Iunius]]
|colspan="2"| Month of [[Juno (mythology)|Juno]]
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|-
|style="text-align:right"| 7. ||colspan="2"| July
|style="text-align:right"| VII. ||colspan="2"| [[Mensis Quintilis]]
|colspan="2"| Fifth Month (from the earlier calendar starting in March)
|style="text-align:center"| 31
|style="text-align:center"| 31
|style="text-align:center"| 31
|style="text-align:center"| 31
|-
|style="text-align:right"| 8. ||colspan="2"| August
|style="text-align:right"| VIII. ||colspan="2"| [[Mensis Sextilis]]
|colspan="2"| Sixth Month
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|-
|style="text-align:right"| 9. ||colspan="2"| September
|style="text-align:right"| IX. ||colspan="2"| [[September (Roman month)|Mensis September]]
|colspan="2"| Seventh Month
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|-
|style="text-align:right"| 10. ||colspan="2"| October
|style="text-align:right"| X. ||colspan="2"| [[October (Roman month)|Mensis October]]
|colspan="2"| Eighth Month
|style="text-align:center"| 31
|style="text-align:center"| 31
|style="text-align:center"| 31
|style="text-align:center"| 31
|-
|style="text-align:right"| 11. ||colspan="2"| November
|style="text-align:right"| XI. ||colspan="2"| [[November (Roman month)|Mensis November]]
|colspan="2"| Ninth Month
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|-
|style="text-align:right"| 12. ||colspan="2"| December
|style="text-align:right"| XII. ||colspan="2"| [[December (Roman month)|Mensis December]]
|colspan="2"| Tenth Month
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|style="text-align:center"| 29
|-
!scope="row" colspan="8" style="text-align:right"| Whole year:
|style="text-align:center"| 355
|style="text-align:center"| 377
|style="text-align:center"| 355
|style="text-align:center"| 378
|}

According to the later writers Censorinus and Macrobius, to correct the mismatch of the correspondence between months and seasons due to the excess of one day of the Roman average year over the tropical year, the insertion of the intercalary month was modified according to the scheme: common year (355 days), leap year with 23-day February followed by 27-day Mercedonius (377 days), common year, leap year with 23-day February followed by 28-day Mercedonius (378 days), and so on for the first 16 years of a 24-year cycle. In the last 8 years, the intercalation took place with the month of Mercedonius only 27 days, except the last intercalation which did not happen. Hence, there would be a typical common year followed by a leap year of 377 days for the next 6 years and the remaining 2 years would sequentially be common years. The result of this twenty-four-year pattern was of great precision for the time: 365.25 days, as shown by the following calculation:

<math>\frac{355\times 13+377\times 7+378\times 4}{24}=\frac{8,766}{24}=365\tfrac{1}{4}</math>

The consuls' terms of office were not always a modern calendar year, but ordinary consuls were elected or appointed annually. The traditional [[list of Roman consuls]] used by the Romans to date their years began in 509 BC.<ref name=matlock>{{harvp|Mathieson|2003|p=[https://books.google.com/books?id=krcp3GU2MssC&pg=PA14 14]}}.</ref>

{{anchor|Gnaeus Flavius}}

===Flavian reform===
[[Gnaeus Flavius (jurist)|Gnaeus Flavius]], a secretary (''scriba'') to censor [[Appius Claudius Caecus|App. Claudius Caecus]], introduced a series of reforms in 304 BC.{{sfnp|Michels|1949|p=340}} Their exact nature is uncertain, although he is thought to have begun the custom of publishing the calendar in advance of the month, depriving the priests of some of their power but allowing for a more consistent calendar for official business.{{sfnp|Lanfranchi|2013}}

{{anchor|Julius Caesar|Caesar|Julian calendar}}

===Julian reform===
{{main|Julian calendar}}
[[Julius Caesar]], following his [[Battle of Pharsalia|victory]] in [[Caesar's Civil War|his civil war]] and in his role as ''[[pontifex maximus]]'', ordered a [[Julian reform|reformation of the calendar]] in 46 BC. This was undertaken by a group of scholars apparently including the [[Alexandria]]n [[Sosigenes of Alexandria|Sosigenes]]{{sfnp|Pliny|loc=Book XVIII, Ch. 211}} and the Roman M. Flavius.{{sfn|Macrobius|loc=Book I, Ch. 14, §2}}{{sfnp|Kaster|2011}} Its main lines involved the insertion of ten additional days throughout the calendar and regular intercalation of a single [[leap day]] every fourth year to bring the Roman calendar into close agreement with the solar year. The year 46 BC was the last of the old system and included three intercalary months, the first inserted in February and two more—''{{lang|la|Intercalaris Prior}}'' and ''{{lang|la|Posterior}}''—before the kalends of December.

===Later reforms===
{{main|Byzantine calendar}}
After [[Assassination of Julius Caesar|Caesar's assassination]], [[Mark Antony]] had Caesar's birth month Quintilis renamed [[July (month)|July]] (''{{lang|la|Iulius}}'') in his honor. After Antony's [[Battle of Actium|defeat at Actium]], [[Augustus]] assumed control of Rome and, finding the priests had (owing to their inclusive counting) been intercalating every third year instead of every fourth, suspended the addition of leap days to the calendar for one or two decades until its proper position had been restored. See [[Julian calendar#Leap year error|Julian calendar: Leap year error]]. In 8 BC, the [[plebiscite]] ''Lex Pacuvia de Mense Augusto'' renamed Sextilis [[August (month)|August]] (''{{lang|la|Augustus}}'') in his honor.{{sfnp|Rotondi|1912|p=441}}{{sfn|Macrobius|loc=Book I, Ch. 12}}{{sfnp|Kaster|2011}}{{efn|There are some documents which state the month had been renamed as early as 26 or 23 BC, but the date of the Lex Pacuvia is certain.}}

In large part, this calendar continued unchanged under the [[Roman Empire]]. ([[Roman Egypt|Egypt]]ians used the related [[Alexandrian calendar]], which Augustus had adapted from [[Egyptian calendar|their wandering ancient calendar]] to maintain its alignment with Rome's.) A few emperors altered the names of the months after themselves or their family, but such changes were abandoned by their successors. [[Diocletian]] began the 15-year [[indiction]] cycles beginning from the AD 297 census;<ref name=matlock/> these became the required format for official dating under [[Justinian I|Justinian]]. [[Constantine the Great|Constantine]] formally established the 7-day [[week]] by making [[Sunday]] an official holiday in 321.{{Citation needed|date=November 2019}} Consular dating became obsolete following the abandonment of appointing nonimperial consuls in AD 541.<ref name=matlock/> The Roman method of numbering the days of the month never became widespread in the Hellenized eastern provinces and was eventually abandoned by the [[Byzantine Empire]] in [[Byzantine calendar|its calendar]].

{{anchor|Kalends|Nones|Ides}}