Roman calendar

The earliest Roman calendar appears to have been lunar, with 10 lunar months. According to tradition, in 509 bce the king Numa Pompilius replaced that calendar with one more like that of the Greeks: a year of 12 months and 355 days. Those months which had 30 days in the old calendar now had 29. Januarius (29 days), named after the god of gates, was added to the beginning of the year and Februarius (28 days), added to the end. In 450 bce, Januarius was moved from before Martius to before Februarius, which established the order of months we still use.

Name of month	Derivation of Name	Number of days
Martius	from the god Mars	31
Aprilis	not known	29
Maius	not known	31
Iunius	from the goddess Juno, wife of Zeus	29
Quintilis	fifth	31
Sextilis	sixth	29
September	seventh	29
October	eighth	31
November	ninth	29
December	tenth	29
Januarius	from Janus, god of gates	29
Februarius	from Februa, a goddess of purification	28

The year began on Martius 1, the day on which new consuls were inaugurated. In 154 bce a rebellion broke out in Spain. To avoid changing consuls in the middle of a war, New Year's was shifted up two months, to Januarius 1, beginning Januarius 1, 153 bce.

Since the total number of days in a year of these months was less than a solar year, an extra month 22 or 23 days long, the mensis intercalaris, called Mercedonis, was added every two to four years after Februarius 23, followed by the last five days of Februarius. This addition had to be called for by the chief priest, the Pontifex Maximus. Our practice of adding the extra day for leap year in February, rather than December, is a survival of the Roman calendar.

Generally Romans identified a year by the names of the consuls elected for that year.¹ Later they began to count years from the supposed date of the founding of the city, ab urbe condita (auc). In doing so they introduced the concept of an era to Western calendars. The beginning of the auc era was placed by the Roman historian Marcus Terentius Varro (116–27 bce) at what we would call 753 bce.

The problem with the Roman calendar was that it was highly adjustable and the adjusters did not show any sense of astronomical discipline. Politicians, for example, sometimes succeeded in lengthening months to prevent their term of office from expiring at a critical point, or the chief priest failed to add the necessary mensis intercalaris at the right time. The result was that by the time Julius Caesar overthrew the Republic the calendar was three months out of whack with the seasons. See Julian calendar.

Roman days of the month

The Romans had a complicated way of stating what day of the month it was. It depended on three “milestone” days that occurred in every month:

Kalends	the first day of the month (originally, the day of the new moon)
Nones	8 days before the Ides
Ides	the 15th day of Martius, Maius, Julius (originally Quintilis) and October; the 13th day of other months (originally, the Ides was the day of the full moon)

To specify a date, the Romans counted backwards from the next of these “milestones.” So, for example, December 2 would be “ante diem IV nonas Decembres.” Days after the Ides are given as days before the Kalends of the next month; that is, what we would call January 29 they called “4 days before the First of February.”

Notice that in a truly lunar calendar, counting down to the next full moon (or new moon) is an eminently practical way of naming the days of the month.

Another term encountered in the names of many days is “pridie,” which means “the day before.” Thus “pridie idus Martias” is the day before the ides of March.

This system was used in Europe as late as the 16^th century.

The names of the days can be read from the following table. In writing, the names were usually abbreviated; the abbreviations are self-evident.

Roman Names of the Days of the Month
where M = adjectival form of the name of the current month;
N = adjectival form of the name of the following month
Day of the month	Month (adjectival form)
Day of the month	Ianuarias, Augustas, Decembres	Februarias (not a leap year)	Februarias (leap year)	Martias, Maias, Iulias, October	Apriles, Iunias, Septembres, Novembres
1	kalendis M*	kalendis M*	kalendis M*	kalendis M*	kalendis M*
2	ante diem IV nonas M	ante diem IV nonas M	ante diem IV nonas M	ante diem VI nonas M	ante diem IV nonas M
3	ante diem III nonas M	ante diem III nonas M	ante diem III nonas M	ante diem V nonas M	ante diem III nonas M
4	pridie nonas M	pridie nonas M	pridie nonas M	ante diem IV nonas M	pridie Nones
5	nonis M*	nonis M*	nonis M*	ante diem III nonas M	nonis M*
6	ante diem VIII idus M	ante diem VIII idus M	ante diem VIII idus M	pridie nonas M	ante diem VIII idus M
7	ante diem VII idus M	ante diem VII idus M	ante diem VII idus M	nonis M*	ante diem VII idus M
8	ante diem VI idus M	ante diem VI idus M	ante diem VI idus M	ante diem VIII idus M	ante diem VI idus M
9	ante diem V idus M	ante diem V idus M	ante diem V idus M	ante diem VII idus M	ante diem V idus M
10	ante diem IV idus M	ante diem IV idus M	ante diem IV idus M	ante diem VI idus M	ante diem IV idus M
11	ante diem III idus M	ante diem III idus M	ante diem III idus M	ante diem V idus M	ante diem III idus M
12	pridie idus M	pridie idus M	pridie idus M	ante diem IV idus M	pridie idus M
13	idis M*	idis M*	idis M*	ante diem III idus M	idis M*
14	ante diem XIX kalendas N	ante diem XVI kalendas N	ante diem XVI kalendas N	pridie idus M	ante diem XVIII kalendas N
15	ante diem XVIII kalendas N	ante diem XV kalendas N	ante diem XV kalendas N	idis M*	ante diem XVII kalandas N
16	ante diem XVII kalendas N	ante diem XIV kalendas N	ante diem XIV kalendas N	ante diem XVII kalendas N	ante diem XVI kalendas N
17	ante diem XVI kalendas N	ante diem XIII kalendas N	ante diem XIII kalendas N	ante diem XVI kalendas N	ante diem XV kalendas N
18	ante diem XV kalendas N	ante diem XII kalendas N	ante diem XII kalendas N	ante diem XV kalendas N	ante diem XIV kalendas N
19	ante diem XIV kalendas N	ante diem XI kalendas N	ante diem XI kalendas N	ante diem XIV kalendas N	ante diem XIII kalendas N
20	ante diem XIII kalendas N	ante diem X kalendas N	ante diem X kalendas N	ante diem XIII kalendas N	ante diem XII kalendas N
21	ante diem XII kalendas N	ante diem IX kalendas N	ante diem IX kalendas N	ante diem XII kalendas N	ante diem XI kalendas N
22	ante diem XI kalendas N	ante diem VIII kalendas N	ante diem VIII kalendas N	ante diem XI kalendas N	ante diem X kalendas N
23	ante diem X kalendas N	ante diem VII kalendas N	ante diem VII kalendas N	ante diem X kalendas N	ante diem IX kalendas N
24	ante diem IX kalendas N	ante diem VI kalendas N	ante diem VI kalendas N	ante diem IX kalendas N	ante diem VIII kalendas N
25	ante diem VIII kalendas N	ante diem V kalendas N	ante diem bis VI kalendas N	ante diem VIII kalendas N	ante diem VII kalendas N
26	ante diem VII kalendas N	ante diem IV kalendas N	ante diem V kalendas N	ante diem VII kalendas N	ante diem VI kalendas N
27	ante diem VI kalendas N	ante diem III kalendas N	ante diem IV kalendas N	ante diem VI kalendas N	ante diem V kalendas N
28	ante diem V kalendas N	pridie kalendas N	ante diem III kalendas N	ante diem V kalendas N	ante diem IV kalendas N
29	ante diem IV kalendas N	—	pridie kalendas N	ante diem IV kalendas N	ante diem III kalendas N
30	ante diem III kalendas N	—	—	ante diem III kalendas N	pridie kalendas N
31	pridie kalendas N	—	—	pridie kalendas N	—

* For these "milestone" days the name of the month would be in the ablative plural, i.e., Ianuariis, Februariis, Martiis, Apriliis, Maiis, Iuniis, Iuliis, Augustiis, Septembriis, Octobriis, Novembriis, Decembriis.

1. For a list of the consuls and their dates, see Christopher Mackay's webpage: www.ualberta.ca/~csmackay/Consuls.List.html

want more?

Agnes Kirsopp Michels.
The Calendar of the Roman Republic.
Princeton, NJ: Princeton University Press, 1967.

Reprinted by the Greenwood Press (Westport, CT) in 1978.

Alan Edouard Samuel.
Greek and Roman Chronology: Calendars and Years in Classical Antiquity.
Munich: Beck, 1972.

From the series Handbuch der Altertumswissenschaft; 1 Abteilung, 7 T.