Notes on the Roman Calendar

The History of the Republican Calendar before Julius Caesar

Traditional starting date for Roman calendar: 753 BC, founding of the city by Romulus; much later styled as AUC: ab urbe condita
Ancient calendar had 10 lunar months, together with a dark unnamed period in the winter:

Martius (Mars – time to go to war)
Aprilis (Aphrodite?)
Maius (mother of Mercury)
Junius[1] (Juno)
Quintilis
Sextilis
September
October
November
December
60 nameless days

King Numa added Januarius (Janus) and Februarius (purification by sacrifice). The year began in January. This was a lunar calendar with 355 days (even is unlucky), with 31 or 29 days in each month, except unlucky February (28 days). Martius, Maius, Quintilis and October had 31 days, the others had 29. 4(31)+7(29)+28=355.
He had an extra month with 27 days periodically intercalated, called Mercedonis (wages). This month was added after the 23^rd or 24^th of February, and the rest of February skipped. This gave a year of 377 or 378 days.
Intercalations were viewed with suspicion, and fell prey to corruption and political intrigue. Intercalations were to announced by the pontifex maximus – an office Caesar himself held before he consolidated power as dictator. Consequently, the calendar did not stay synchronized well with either the moon or the sun.

The Roman Labeling of the Days of the Month

Each month has three marker days, from which all other days are counted –backwards and inclusively! These marker days had their origin in the phases of the moon, although they long since lost any temporal connection with the actual moon.
The first day of the month is the Kalends – from which we get our word ‘calendar’. The word means ‘proclaim’, and originally announced the new moon.
The hollow or first quarter moon was associated with the 5^th day of 29 day months, and on the 7^th day of 31 day months; this was called the Nones, which means the ninth day counted inclusively and backwards from the Ides.
The Ides, meaning `division’, was associated with the full moon, and occurred on the 13^th or 15^th of the month, depending on whether the month has 29 or 31 days.
All remaining days are counted from these markers. We summarize these counts in the table below:

Month names	Martius, Maius, Quintilis, October	Other Months	February
Days	31	29	28
1	Kalends	Kalends	Kalends
2	a.d. VI Non.	a.d. IV Non.	a.d. IV Non.
3	a.d. V Non.	a.d. III Non.	a.d. III Non.
4	a.d. IV Non.	pridie Non.	pridie Non.
5	a.d. III Non.	Nones	Nones
6	Pridie Non.	a.d. VIII Idus	a.d. VIII Idus
7	Nones	a.d. VII Idus	a.d. VII Idus
8	a.d. VIII Idus	a.d. VI Idus	a.d. VI Idus
9	a.d. VII Idus	a.d. V Idus	a.d. V Idus
10	a.d. VI Idus	a.d. IV Idus	a.d. IV Idus
11	a.d. V Idus	a.d. III Idus	a.d. III Idus
12	a.d. IV Idus	Pridie Idus	Pridie Idus
13	a.d. III Idus	Ides	Ides
14	Pridie Idus	a.d. XVII Kal.	a.d. XVI Kal.
15	Ides	a.d. XVI Kal.	a.d. XV Kal.
16	a.d. XVII Kal.	a.d. XV Kal.	a.d. XIV Kal.
17	a.d. XVI Kal.	a.d. XIV Kal.	a.d. XIII Kal.
18	a.d. XV Kal.	a.d. XIII Kal.	a.d. XII Kal.
19	a.d. XIV Kal.	a.d. XII Kal.	a.d. XI Kal.
20	a.d. XIII Kal.	a.d. XI Kal.	a.d. X Kal.
21	a.d. XII Kal.	a.d. X Kal.	a.d. IX Kal.
22	a.d. XI Kal.	a.d. IX Kal.	a.d. VIII Kal.
23	a.d. X Kal.	a.d. VIII Kal.	a.d. VII Kal.
24	a.d. IX Kal.	a.d. VII Kal.	a.d. VI Kal.
25	a.d. VIII Kal.	a.d. VI Kal.	a.d. V Kal.
26	a.d. VII Kal.	a.d. V Kal.	a.d. IV Kal.
27	a.d. VI Kal.	a.d. IV Kal.	a.d. III Kal.
28	a.d. V Kal.	a.d. III Kal.	Pridie Kal.
29	a.d. IV Kal.	Pridie Kal.
30	a.d. III Kal.
31	Pridie Kal.

Note that an expression like a.d. IV Kal. would be written out as ante diem IV Kalendae (next month name). After Julius Caesar’s reform, the thirty-day months followed the basic format of the 29 day months.

The days in the Roman calendar were also labeled by a sequence of letters A-H; these eight labels specified the kind of business or activity permissible for each day. The market days occurred every ninth day (by inclusive counting), and were known as nunidae. Kalends Jan. was always labeled A and printed in red: a red letter day. Later this was adopted in the Christian tradition, by printing festival days in red.

Julius Caesar’s Reform

Calendar in serious disarray – partly because the pontifex Caesar was out of Rome fighting his wars.
In 50 BC Cicero didn’t know whether there would be an intercalary month – 3 days before the fact!
The calendar was about 3 months out of step.
Caesar returned in triumph in 47 BC, having defeated Pompey. Vainglorious games in celebration.
Caesar’s principles:

Abandon the moon.
Abandon intercalations.
Adopt a basic 365 day year.
Have a leap day every fourth year – added after February 23^rd.
Alter the number of days of the months – 31 or 30 (28 for February).
Bring the year into alignment with the seasons, by a one-time addition of 90 days to 46 BC – “ultimus annus confusionis”.

When the leap day was added, it was known as ante diem bis VI Kal. Mar. – the second sixth day before Kalends Mar. This is why leap years are known as bissextile years.
Caesar was assassinated on the Ides of March, 44BC. Confusion, Civil War – leading to Emperor Caesar Augustus.
Quintilis named for Julius; confusion about whether the 4 year count was inclusive or not. Augustus repaired it by skipping some leap years until 8 AD. Sextilis named for Augustus. Tiberius turns down yet another month named after him.

[1] I will use the letter J consistently, even though the Romans would have written Iunius.