The Roman calendar

Commemorative coin in honor of
Numa Pompilius

According to Plutarch, the Roman calendar was established by the second king of Rome, Numa Pompilius (753-674 BC), who at first divided the year into ten months, beginning in March, and gave numerical names to the fifth to tenth months, but later added two extra months (January and February), and moved the beginning of the year to January 1st. The months of the early Roman calendar, therefore, were these: Ianuarius, Februarius, Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November and December. It will be noted that, by adding two months at the beginning, the numbers of the fifth to tenth months became seventh to twelfth, but the names were already fixed and nobody bothered to correct them and adapt to the new situation. Plutarch comments on the origin of the month names:

The first month, consecrated by Romulus to Mars, was called Martius, and the second Aprilis, named after Aphrodite, who is Venus, because in this month sacrifices are made to this Goddess... The next month is called Maius, after Maia, as it is devoted to Mercury [son of Maia]; and Iunius is named after the goddess Juno. But there are some who argue that they take their denomination from the oldest and the youngest; because the eldest are called maiores, and the youngest iuniores... The first, Ianuarius, comes from Janus [the god of the doors].

The Roman months were lunar, alternating 28 and 29 days. As twelve lunar months fall short of the year by more than 11 days, from time to time an additional month was added (the thirteenth month), but a regular system was not established for the addition, as they did in Babylon and Greece. The decision to add the additional month was taken by the pontifex maximus, the main religious authority. But this position was political and fell under the party game, which was especially virulent in the last years of the republic. As the political magistracies lasted a year, the additional month was inserted when the pontifex wished to prolong the government of the party holding power, and omitted it when the magistrates belonged to the opposite party. The result was chaotic. By mid-first century B.C., the total error amounted to eighty days, almost a season.

Julius Caesar

The dictator Caius Julius Caesar put an end to this state of affairs. To adjust the Roman year to the seasons, he added three months to the year 46 B.C., which thus had 445 days and was called the year of confusion, but ended with an unsustainable situation. He then imposed a reform of the calendar, taking as a model the Egyptian solar year, with the help of Sosigenes, an astronomer from Alexandria. The new Roman calendar, called Julian in honor of its sponsor, was even better than the Egyptian. The duration of the solar year had been calculated to be 365 days and a quarter. Sosigenes took it into account and designed the Julian calendar as a four-year cycle: three years with 365 days, and one with 366, by adding an extra day.

The adaptation of the months to the phases of the moon was abandoned: the year was divided into twelve months of 30 and 31 days, with the exception of February, which was assigned 29 (see table). The Romans considered February as a month of bad omen (it was the month of the dead and of the infernal gods) and for that reason they made it shorter. On the other hand, with this distribution, the years with 366 days were completely regular, by alternating months of 31 and 30 days.

Month	Days (normal yr.)	Days (leap yr.)	Month	Days (normal yr.)	Days (leap yr.)
Ianuarius	31	31	Quintilis	31	31
Februarius	29	30	Sextilis	30	30
Martius	31	31	September	31	31
Aprilis	30	30	October	30	30
Maius	31	31	November	31	31
Iunius	30	30	December	30	30

Number of days in each month after the calendar reform by Julius Caesar.

The extra day in the 366-day years was inserted in the month of February, between the seventh and sixth day before the March Calends, i.e. between February 24 and 25 according to our current numbering. For this reason they called this day bis sexto calendae Martii (second sixth day before the March Calends), and the common name leap day has been transferred to the entire year.

Augustus

Julius Caesar was murdered on the Ides of March of 44 BC, about a year after his calendar was introduced. In recognition, the Senate dedicated him the month of his birth (Quintilis), renamed Iulius. Shortly thereafter, an error of interpretation led to insert leap days every three years rather than four. In 8 B.C. Augustus corrected this error. Consequently, the Senate also dedicated him a month, choosing Sextilis, which was renamed Augustus. When the Senate offered his successor to dedicate him September, Tiberius rejected the offer by saying: What will you do after the twelfth emperor?

As July had 31 days, Augustus’s month couldn’t be less, therefore an additional day was assigned to it, which was removed from February. In addition, since there would be three consecutive months with 31 days (July, August and September), the distribution of the last four months of the year was modified, giving rise to the following table, which has remained unchanged till today. After its correction by Augustus, the Julian calendar was applied for over a millennium and a half.

Month	Days (normal yr.)	Days (leap yr.)	Month	Days (normal yr.)	Days (leap yr.)
Ianuarius	31	31	Iulius	31	31
Februarius	28	29	Augustus	31	31
Martius	31	31	September	30	30
Aprilis	30	30	October	31	31
Maius	31	31	November	30	30
Iunius	30	30	December	31	31

Number of days in each month after the calendar reform by Augustus.

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Manuel Alfonseca