The earth orbits the sun at roughly 365¼ days a year.
But before 45 BC, the Roman calendar consisted of just 355 days. A 27-day intercalary month, the Mensis Intercalaris, was sometimes inserted between February and March, adding an extra twenty-two or twenty-three days to the year, as a catch up.
As far as historians can determine, these extra months were added every second or third year. If managed correctly this system allowed the Roman year to stay roughly aligned to the seasons.
However, the extra month had to be approved by a Roman magistrate, a Pontifix, and because terms of office corresponded with a calendar year, this power was prone to abuse. A Pontifex could lengthen a year in which he or one of his allies was in power, or refuse to lengthen one in which his opponents held sway.
By the time of Julius Caesar, the system had failed completely and the average Roman was often unsure of the date. The last years of the pre-Julian calendar became known as “The Years of Confusion”.
It prompted Caesar to develop a new calendar that could be aligned to the sun without any human intervention – to keep the smooth running of the country and its agriculture free from politiciking, in other words.
It became effective in 45 BC, distributing an extra ten days among the months of the Roman Republican year. It also added an extra day every four years – what was called a leap year to account for that pesky quarter day rotation.
According to Cicero, in order to align the year with the seasons, Caesar had to insert two extra months called the Intercalaris Prior and the Intercalaris Posterior just before December.
So that year had three Novembers.
I learned all this while researching my novel, Cleopatra, When We Were Gods:
“Did you hear about Marcellus?” Cicero said. “He was seen coming out of a brothel near the Circus Maximus last week. His wife was furious with him but he countered her by saying he could do as he liked as the first two Novembers of the year didn’t count.”
“I heard Lepidus is thrown in a panic,” Antony said. “He is writing everything down that he does each day, so that he can do exactly the same thing on the corresponding days in the two Novembers to follow.”
Thanks to the three Novembers, Caesar’s calendar was the predominant calendar in the western world, for more than 1600 years.
But there was a flaw.
The earth’s orbit is not 365¼ days; it is 365.2425 days.
In 1582, to accommodate this slight difference, Pope Gregory effected a modification, now called the Gregorian calendar, which is what we use today.
Years that were evenly divisible by 100 were not leap years, unless they were also evenly divisible by 400. For example, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. Similarly 2100 will not be a leap year.
This has to happen, otherwise because of solar drift, today would be two weeks ago.
Caesar’s improvements to the calendar were revolutionary, and very clever, but they didn’t help poor Julius. The year after he brought in the changes he was slaughtered outside the Senate.
But in March, not November.