In beginners’ Latin, we recently did our first lesson on Roman numbers, and a student asked me why the names of the last four months, the ninth to twelfth, derive from the words “septem – seven”, “octo – eight”, etc. This comes from the ancient calendar which the Romans themselves attributed to their founder Romulus, which had only 304 days, distributed over ten months, from March to December. September was therefore originally the seventh month, October the eight, and so on. The names “July” and “August” come from Julius Caesar and Augustus, but originally these months were called “Quintilis” and “Sextilis”, i.e., the fifth and sixth month. The period which we now call January and February was simply not counted, and indeed, had no name.
The inconvenience of such a system is obvious, and Roman tradition ascribed the creation of these latter two months, along with many other religious institutions, to Romulus’ immediate successor as king of Rome, Numa Pompilius. However, the calendar of Romulus was lunar, rather than solar, and even as modified by Numa, counted only 355 days, ten less than the standard solar year, and not including the extra quarter day now counted by the leap years. This discrepancy would quickly bring the calendar wildly out of harmony with the seasons; religious festivals tied to a particular date on the calendar, but observed because of the time of year or an agricultural season, would then occur well off their appropriate dates.
This problem was supposed to be remedied by the addition of an extra month after February, which was called “Mercedonius” or “Mercedinus.” This was not, however, added every year on a regular basis. Instead, it was proclaimed by the Roman chief priest, the pontifex maximus, as deemed necessary by observation of the seasons; Latin has a special verb “intercalare”, which means to proclaim that such a month will be added, from which derives the English word “intercalary.” Mercedonius was either 22 or 23 days long, the discrepancy between two solar and two lunar years, with allowance for a leap day. The faultiness of this arrangement is demonstrated by the fact that when a total eclipse of the sun took place on March 14, 190 BC, the day was recorded in Rome as “the fifth day before the Ides of July”, i.e. July 11.
The Romans also had other way of harmonizing the solar and lunar calendars; in 1915, a painted wall-calendar of the late Republican period was found, which has a mensis intercalaris of 26 days after December.
(A reconstruction of the aforementioned calendar, which is known as the Fasti Antiates Maiores; image from Wikimedia Commons by Bauglir, CC BY-SA 4.0)
Those who held the office of the pontifex maximus could easily abuse this system by shorten the years in which their political opponents held annually elected offices, and lengthening the years when such offices were held by their political allies. It was therefore definitively reformed by Julius Caesar, who instituted the annual leap day at the end of February (of which we will say more in a week’s time), and abolished the intercalary month of Mercedonius.