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 16th 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)
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.

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