Claus Tøndering –
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The Roman calendar

What is the Roman calendar?

The Roman calendar provides the historical background for the Christian calendar. To a large extent the structure the calendar we use today is similar to the structure of the ancient Roman calendar.

Before 45 BC, the Roman calendar was a mess, and much of our so-called “knowledge” about it seems to be little more than guesswork.

Originally, the year started on 1 March and consisted of only 304 days or 10 months (Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December). These 304 days were followed by an unnamed and unnumbered winter period. The Roman king Numa Pompilius (c. 715-673 BC, although his historicity is disputed) allegedly introduced February and January (in that order) between December and March, increasing the length of the year to 354 or 355 days. In 450 BC, February was moved to its current position between January and March.

In order to make up for the lack of days in a year, an extra month, Intercalaris or Mercedonius, (allegedly with 22 or 23 days though some authorities dispute this) was introduced in some years. In an 8 year period the length of the years were:

1:12 months or 355 days
2:13 months or 377 days
3:12 months or 355 days
4:13 months or 378 days
5:12 months or 355 days
6:13 months or 377 days
7:12 months or 355 days
8:13 months or 378 days

A total of 2930 days corresponding to an average of 366¼ days per year. This year was discovered to be too long, and therefore 7 days were later dropped from the 8th year, yielding 365.375 days per year.

This is all theory. In practice it was the duty of the priesthood to keep track of the calendars, but they failed miserably, partly due to ignorance, partly because they were bribed to make certain years long and other years short. Furthermore, leap years were considered unlucky and were therefore avoided in time of crisis, such as the Second Punic War.

In order to clean up this mess, a calendar reform was required. In 45 BC Julius Caesar introduced the so-called Julian calendar, but before the reformed calendar could be used, drastic measures were required to make up for the many omitted leap months. Therefore 46 BC became a year with 15 months and 445 days; that year has been aptly termed “the last year of confusion”.

The table below shows an educated guess about the length of the months in the years 47 BC (which was a typical Roman year) and 46 BC (which was anything but typical).

47 BC46 BC
Januarius 2929
Februarius 2824
Intercalaris 27
Martius 3131
Aprilis 2929
Maius 3131
Junius 2929
Quintilis 3131
Sextilis 2929
September 2929
October 3131
November 2929
* 33
* 34
December 2929

The names of the two extra months between November and December are not known. It has been suggested that their names were Undecember and Duodecember, but that is doubtful, as this would mean that the names of the last four months were derived from the Latin words for nine, eleven, twelve, ten – in that order.

The length of the months from 45 BC onward were the same as the ones we know today.

Occasionally one reads the following story:

“Julius Caesar made all odd numbered months 31 days long, and all even numbered months 30 days long (with February having 29 days in non-leap years). In 44 BC Quintilis was renamed ‘Julius’ (July) in honour of Julius Caesar, and in 8 BC Sextilis became ‘Augustus’ in honour of emperor Augustus. When Augustus had a month named after him, he wanted his month to be a full 31 days long, so he removed a day from February and shifted the length of the other months so that August would have 31 days.”

This story, however, has no basis in actual fact. It is a fabrication, possibly invented by the English-French scholar Johannes de Sacrobosco in the 13th century.[1]

How did the Romans number days?

The Romans didn’t number the days sequentially from 1. Instead they had three fixed points in each month:

(or “Calendae”), which was the first day of the month.
which was the 13th day of January, February, April, June, August, September, November, and December, or the 15th day of March, May, July, or October.
which was the 9th day before Idus (counting Idus itself as the 1st day).

The days between Kalendae and Nonae were called “the 5th day before Nonae”, “the 4th day before Nonae”, “the 3rd day before Nonae”, and “the day before Nonae”. (There was no “2nd day before Nonae”. This was because of the inclusive way of counting used by the Romans: To them, Nonae itself was the first day, and thus “the 2nd day before” and “the day before” would mean the same thing.)

Similarly, the days between Nonae and Idus were called “the Xth day before Idus”, and the days after Idus were called “the Xth day before Kalendae (of the next month)”.

Julius Caesar decreed that in leap years the “6th day before Kalendae of March” should be doubled. So in contrast to our present system, in which we introduce an extra date (29 February), the Romans had the same date twice in leap years. The doubling of the 6th day before Kalendae of March is the origin of the word bissextile. If we create a list of equivalences between the Roman days and our current days of February in a leap year, we get the following:

Roman dateModern date
7th day before Kalendae of March23 February
6th day before Kalendae of March24 February
6th day before Kalendae of March25 February
5th day before Kalendae of March26 February
4th day before Kalendae of March27 February
3th day before Kalendae of March28 February
The day before Kalendae of March29 February
Kalendae of March1 March

Since the leap day was the “6th day before Kalendae of March”, either 24 or 25 February is still today frequently considered the leap day.

Why did Caesar choose to double the 6th day before Kalendae of March? According to some sources the leap month Intercalaris/Mercedonius of the pre-reform calendar was not placed after February, but inside it, namely between the 7th and 6th day before Kalendae of March. It was therefore natural to have the leap day in the same position.[2]


^ [1] Roscoe Lamont: The Roman Calendar and its Reformation by Julius Caesar, Popular Astronomy 27 (1919), pp. 585–597. Available at http://articles.adsabs.harvard.edu/full/1919PA.....27..579P/0000583.000.html, retrieved 26 August 2011
^ [2] Ibid. p. 583.