Roman calendar

A reproduction of the fragmentary Fasti Antiates Maiores (c. 60 BC), with the seventh and eighth months still named Quintilis ("QVI") and Sextilis ("SEX") and an intercalary month ("INTER") in the far righthand column
Museum of the Roman Theater of Caesaraugusta in Zaragoza, Spain
Another reproduction of the Fasti Antiates Maiores

The Roman calendar was the calendar used by the Roman kingdom and republic. It is often inclusive of the Julian calendar established by the reforms of the dictator Julius Caesar and emperor Augustus in the late 1st century BC and sometimes inclusive of any system dated by inclusive counting towards months' kalends, nones, and ides in the Roman manner. It is usually exclusive of the Alexandrian calendar of Roman Egypt, which continued the unique months of that land's former calendar; the Byzantine calendar of the later Roman Empire, which usually dated the Roman months in the simple count of the ancient Greek calendars; and the Gregorian calendar, which refined the Julian system to bring it into still closer alignment with the solar year and is the basis of the current international standard.

Roman dates were counted inclusively forward to the next of three principal days: the first of the month (the kalends), a day less than the middle of the month (the ides), and eight days—nine, counting inclusively—before this (the nones). The original calendar consisted of 10 months beginning in spring with March; winter was left as an unassigned span of days. These months ran for 38 nundinal cycles, each forming an eight-day week (counted inclusively, hence the name) ended by religious rituals and a public market. The winter period was later divided into two months, January and February. The legendary early kings Romulus and Numa were traditionally credited with establishing this early fixed calendar, which bears traces of its origin as an observational lunar one. In particular, the kalends, nones, and ides seem to have derived from the first sighting of the crescent moon, the first-quarter moon, and the full moon respectively. The system ran well short of the solar year, and it needed constant intercalation to keep religious festivals and other activities in their proper seasons. For superstitious reasons, such intercalation occurred within the month of February even after it was no longer considered the last month.

After the establishment of the Roman Republic, years began to be dated by consulships and control over intercalation was granted to the pontifices, who eventually abused their power by lengthening years controlled by their political allies and shortening the years in their rivals' terms of office. Having won his war with Pompey, Caesar used his position as Rome's chief pontiff to enact a calendar reform in 46 BC, coincidentally making the year of his third consulship last for 446 days. In order to avoid interfering with Rome's religious ceremonies, the reform added all its days towards the ends of months and did not adjust any nones or ides, even in months which came to have 31 days. The Julian calendar was supposed to have a single leap day on 24 February (a doubled VI Kal. Mart.) every fourth year but following Caesar's assassination the priests figured this using inclusive counting and mistakenly added the bissextile day every three years. In order to bring the calendar back to its proper place, Augustus was obliged to suspend intercalation for one or two decades. The revised calendar remaining slightly longer than the solar year, the date of Easter shifted far enough away from the vernal equinox that Pope Gregory XIII ordered its adjustment in the 16th century.


The remains of the Fasti Praenestini

Prehistoric lunar calendar

The original Roman calendar is believed to have been an observational lunar calendar[1] whose months began from the first signs of a new crescent moon. Because a lunar cycle is about ​29 12 days long, such months would have varied between 29 and 30 days. Twelve such months would have fallen 10 or 11 days short of the solar year; without adjustment, such a year would have quickly rotated out of alignment with the seasons in the manner of the Islamic calendar. Given the seasonal aspects of the later calendar and its associated religious festivals, this was presumably avoided through some form of intercalation or through the suspension of the calendar during winter.

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.

Legendary 10-month calendar

The Romans themselves described their first organized year as one with ten fixed months, each of 30 or 31 days.[2][3] Such a decimal division fitted general Roman practice.[4] The four 31-day months were called "full" (pleni) and the others "hollow" (cavi).[a][6] Its 304 days made up exactly 38 nundinal cycles. The system is usually said to have left the remaining 50-odd days of the year as an unorganized "winter", although Licinius Macer's lost history apparently stated the earliest Roman calendar employed intercalation instead[7][8] and Macrobius claims the 10-month calendar was allowed to shift until the summer and winter 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.[9][10]

Later Roman writers credited this calendar to Romulus,[11][12] their legendary first king and culture hero, although this was common with other practices and traditions whose origin had been lost to them. Some scholars doubt the existence of this calendar at all, as it is only attested in late Republican and Imperial sources and apparently supported only by the misplaced names of the months from September to December.[13] 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 be suspicious.[clarification needed][13]

Calendar of Romulus
English Latin Meaning Length
in days
March Mensis Martius Month of Mars 31
April Mensis Aprilis Month of Apru (Aphrodite)[14] 30
May Mensis Maius Month of Maia[15] 31
June Mensis Iunius Month of Juno 30
Quintilis Mensis Quintilis
Mensis Quinctilis[16]
Fifth Month 31
Sextilis Mensis Sextilis Sixth Month 30
September Mensis September Seventh Month 30
October Mensis October Eighth Month 31
November Mensis November Ninth Month 30
December Mensis December Tenth Month 30

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 days. Months were employed secondarily and haphazardly, with some counted as 20 days and others as 35 or more.[17][18]

Republican calendar

The attested calendar of the Roman Republic was quite different. It followed Greek calendars in assuming a lunar cycle of ​29 12 days and a solar year of ​12 12 synodic months (​368 34 days), which align every fourth year after the addition of two intercalary months.[6] The additional two months of the year were January and February; the intercalary month was sometimes known as Mercedonius.[6]

The Romans did not follow the usual Greek practice in alternating 29- and 30-day months and a 29- or 30-day intercalary month every other year. Instead, their 3rd, 5th, 7th, and 10th months[b] had 31 days each; all the other months had 29 days except February, which had 28 days for three years and then 29 every fourth year. The total of these months over a 4-year span differed from the Greeks by 5 days, meaning the Roman intercalary month always had 27 days. Similarly, within each month, the weeks did not vary in the Greek fashion between 7 and 8 days; instead, the full months had two additional days in their first week and the other three weeks of every month ran for 8 days ("nine" by Roman reckoning).[19] Still more unusually, the intercalary month was not placed at the end of the year but within the month of February after the Terminalia on the 23rd (a.d. VII Kal. Mart.); the remaining days of February followed its completion. This seems to have arisen from Roman superstitions concerning the numbering and order of the months.[citation needed] The arrangement of the Roman calendar similarly seems to have arisen from Pythagorean superstitions concerning the luckiness of odd numbers.[19]

These Pythagorean-based changes to the Roman calendar were generally credited by the Romans to Numa Pompilius, Romulus's successor and the second of Rome's seven kings,[citation needed] as were the two new months of the calendar.[20][21][c] Most sources thought he had established intercalation with the rest of his calendar.[citation needed] 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[22] centuries before its development by Babylonian and Greek astronomers.[d] Plutarch's account claims he ended the former chaos of the calendar by employing 12 months totaling 354 days—the length of the lunar and Greek years—and biennial intercalary months of 22 days.[17][18]

Plutarch believed Numa was responsible for placing January and February first in the calendar;[17][18] Ovid states January began as the first month and February the last, with its present order owing to the Decemvirs.[24][25] W. Warde Fowler believed the Roman priests continued to treat January and February as the last months of the calendar throughout the Republican period.[26]

Roman Republican calendar
English Latin Meaning Length
in days
January Mensis Ianuarius Month of Janus 29
February Mensis Februarius Month of the Februa 28
Intercalary Month
Mensis Intercalaris
Month of Wages 23
March Mensis Martius Month of Mars 31
April Mensis Aprilis Uncertain 29
May Mensis Maius Uncertain 31
June Mensis Iunius Month of Juno 29
Quintilis Mensis Quintilis
Mensis Quinctilis[16]
Fifth Month 31
Sextilis Mensis Sextilis Sixth Month 29
September Mensis September Seventh Month 29
October Mensis October Eighth Month 31
November Mensis November Ninth Month 29
December Mensis December Tenth Month 29

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.[29]

Flavian reform

Gnaeus Flavius, a secretary to the pontifex maximus, introduced a series of reforms in 304 BC.[30] 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.[31]

Julian reform

Julius Caesar, following his victory in his civil war and in his role as pontifex maximus, ordered a reformation of the calendar in 46 BC. This was undertaken by a group of scholars apparently including the Alexandrian Sosigenes[32] and the Roman M. Flavius.[33][28] Its main lines involved the insertion of ten additional days throughout the calendar and regular intercalation of a single leap day every fourth year in order 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 3 intercalary months, the first inserted in February and two more—Intercalaris Prior and Posterior—before the kalends of December.

Later reforms

After Caesar's assassination, Mark Antony had Caesar's birth month Quintilis renamed July (Iulius) in his honor. After Antony's 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. In 8 BC, the plebiscite Lex Pacuvia de Mense Augusto caused Sextilis to be renamed August (Augustus) in his honor.[34][35][28][e]

In large part, this calendar continued unchanged under the Roman Empire. (Egyptians used the related Alexandrian calendar, which Augustus had adapted from 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;[29] these became the required format for official dating under Justinian. Constantine formally established the 7-day week by making Sunday an official holiday in 321. Consular dating became obsolete following the abandonment of appointing nonimperial consuls in AD 541.[29] 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 its calendar.

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