The Roman and Julian calendars

The term Roman calendar refers to all the calendars used by the Romans until the creation of the Julian calendar in 45 BC by Julius Caesar.

Little is known about the calendars the Romans used before the reform of the Julian calendar, and it is possible that much of our knowledge on this subject is speculation. However, it is clear that many of the details given by some texts or popular narratives on the Roman calendar, which may contain some truth, are not based on historical facts.

The first Roman calendars

According to legend, the first Roman calendar was introduced by the co-founder of Rome, the mythical figure Romulus, in 753 BC, year 1 of the Roman calendar (AUC: ab urbe condita: since the city was founded).

This calendar would have had 10 months over a year, with a total of 304 days. It is possible that the peoples who lived in Italy around 700 BC used 10-month calendars, but this was certainly not practical. The beginning of the year began at the time of the spring equinox, and after the 10th month, an intermediate winter period was introduced with an ‘appropriate’ duration in order to be not too out of step with the seasons.

The names of these months were: Martius (related to the god Mars), Aprilis (related to Aperta, the nickname of Apollo?), Maius (another name for Jupiter), June (related to the goddess Juno), then Quintilis (5th month), Sextilis (6th), September (7th), October (8th), November (9th), December (10th). But these names may be later ‘inventions’.

Ten, twelve or thirteen months?

Still according to the accounts, the second king of Rome, Numa Pompilius (715-673 BC, but he may have been a mythical figure), the successor of Romulus, added two months to the calendar: Februarius in Januarius. These months are placed before Martius, so that the meaning of the names of the months no longer corresponded to the rank number. Around 450 BC Februarius and Januarius were exchanged and the first month of the year began with Januarius.

The number of days in each month is not entirely clear. According to some, Martius included 31 days, Aprilis 30 and the following months alternately 31 and 30, but September had 30 days, October 31 and November and December 30 each. Numa Pompilius withdrew 1 day of each month that contains 30 days and gave 29 days to the month Januarius and 28 to the month Februarius. The year therefore lasted 355 days.

In order for the seasons to fall at about the same time of the year, an extra month is added at the end of some years (called Intercalaris or Mercedonius with 22 or 23 days, although there are some disagreements among the experts about the duration of this month).

By alternating a year with and without a leap month over an 8-year period and allowing the leap month to last 22 or 23 days, we obtain a total of 2930 days or an average of 366.25 days per year, which was about one day too long compared to the actual period of the Earth’s revolution around the Sun. As a result, 7 days was removed in the 8th year.

However, all of this remains theory. In practice, it was the duty of the great pontiffs, the priests of the time, to follow the calendars and insert the leap months. However, they failed miserably, partly because of ignorance, but also for political (wars, holidays…), financial (transactions, taxes), legal (disputes…) or religious motives (sacrifices, superstition, fear of all even numbers…)

At the time of Julius Caesar, the difference between the calendar year and the solar year was almost two months.

The reform of the Julian calendar

Julius Caesar reformed the calendar, on the advice of several scientists, including Sosigenes of Alexandria. Julius Caesar introduced a 365-day year, with, every four years, a leap year (bissextile year) that contains an extra day in February. From 45 BC, he started the year in January (31 days), followed by February (29 or 30 days in a leap year). The months following February had 30 or 31 days alternately.

Before this reform could be implemented, a number of corrections still had to be made. The year 46 BC was probably the most remarkable year in the history of the calendar.

In that year, February is exceptionally 24 days long. Between February and March, a 27-day leap month was inserted. The months that followed this leap month were, until November similar to those in the old Roman calendar with 29- and 31-day months. After November, two leap months were added so that the 25th of March of the following year coincides with early spring. The names of these months between November and December are not known, but several sources suggested that they were named Undecember (33 days) and Duodecember (34 days). Finally, the year 46 BC ended in December, which is 29 days long. In total, there had been 445 days in this ‘year of confusion’.

After these adjustments, the year 45 BC could start according to the Julian calendar. The month Quintilis has been renamed Julius. In 8 BC, the month Sextilis was renamed Augustus, the name of Rome’s first emperor.

It is not clear when August got 31 days, September and November 30 days, October and December 31 days and February 28 days (or 29 days in leap years). Some sources even claim that these last adjustments were only made in the 14th century.

The days of the month for the Romans

The Romans did not count the days in the same way as in the current Western calendar. While we count our days after the first day of the month (for example, June 22), the Romans counted the days based on three landmark days in the month:

  • the ‘Kalendae’: the first day of the month
  • the ‘Idus’: the 13th day of January, February, April, June, June, June, August, September, November or December, or the 15th day of March, May, July or October. According to other sources, it is also the 15th day of each ‘long month’ (31 days), i.e. the 15th day of January, August and December in the Julian calendar
  • the ‘Nonae’: the 9th day before ‘Idus’ (including Idus)

The days of the month are then counted from these three landmark days. For example, a.d. V Nonae (ante diem) is 4 days before the Nonae (the 3rd of the month). a.d. VI Kalendae is 7 days before the first of the following month. In an ordinary month of February, it corresponds to February 24. It is this day that was traditionally ‘doubled’ in a leap year. Hence the name ‘bissextile’, a sixth day ‘bis’ which was introduced in a leap year (bissextile year). The actual leap day in a bissextile year is therefore February 24 (sometimes February 25).


You can find detailed information on calendars in The Calender FAQ but also on hundreds of other pages on the Internet. Keep in mind the text of the first two paragraphs of this page.