A calendar with a legend attached
Our month names come from Rome, and some of them preserve numbers that no longer match their position. September, October, November and December come from the Latin words for seven, eight, nine and ten, yet they are now the ninth to twelfth months. That mismatch points to an older calendar in which the year began in March. The story of how Rome got from that calendar to the twelve-month, 365-day year of Julius Caesar is partly well documented and partly legend, and it is best to separate the two.
The traditional ten-month year
Roman writers of later centuries described an early calendar attributed to Romulus, the legendary first king. It had ten months, beginning with Martius (March) and ending with December, for a total of 304 days. Four months of thirty-one days and six months of thirty days add up to that figure: 4 times 31 is 124 and 6 times 30 is 180, giving 304.
The winter gap of about sixty days was, in this account, simply left uncounted. Modern historians are cautious about this picture. It is possible that the early Roman year was always more complicated, or that the ten-month description was a later explanation for names such as September. What can be said is that the names of the months from the fifth to the tenth, Quintilis, Sextilis, September and so on, are ordinal numbers, which supports the idea that a count beginning in March was at one time in use. These number-derived words are a good example of how names preserve older systems Quintilis and Sextilis, for example, are built from the Latin ordinal numbers for “fifth” and “sixth”.
Numa and the twelve-month year
Tradition credits the second king, Numa Pompilius, with adding January and February to bring the year to twelve months. The republican calendar of later times had 355 days, reckoned as four months of 31 days (March, May, Quintilis and October), seven months of 29 days and February with 28: 4 times 31 is 124, 7 times 29 is 203, and 124 plus 203 plus 28 gives 355.
The lengths show a Roman preference for odd numbers. All the months except February had an odd number of days, a pattern linked in ancient sources to the belief that odd numbers were more fortunate, a belief described under odd number luck. February, with its even 28 days, was associated with rites of purification and the dead.
A 355-day year is about ten days short of the solar year, so an extra month, Mercedonius, was supposed to be inserted every second year, alternately of 22 and 23 days, after 23 February. With this scheme the average year over four years was 366.25 days, a little too long.
The calendar in daily use
The Romans did not number the days of a month from first to last. They counted backwards from three fixed points: the Kalends (the first of the month), the Nones (the fifth or seventh) and the Ides (the thirteenth or fifteenth). The Ides fell on the 15th in March, May, July and October and on the 13th in other months. This is why the Ides of March means 15 March. A date such as “the third day before the Kalends of April” was written with the Roman numerals described elsewhere on the site.
The roles of days also mattered in civic life. Certain days were designated for public business and others were not, which gave the priests, called pontiffs, great influence over the calendar.
Why it drifted
The scheme relied on the pontiffs deciding when to insert Mercedonius. Because the insertion could extend or shorten the term of office of magistrates, it sometimes served political purposes, and in periods of civil disorder it was neglected. By the middle of the first century BCE the calendar had slipped badly out of step with the seasons.
Julius Caesar’s reform
In 46 BCE, Julius Caesar, acting as pontifex maximus, introduced a new system. Ancient sources say he consulted the Alexandrian astronomer Sosigenes. To bring the calendar back in step, that year was made much longer. It is reported as being about 445 days, formed by the usual 355-day year, Mercedonius, and two further inserted months: roughly 355 plus 23 plus 67 equals 445. Ancient writers called it the year of confusion.
The new Julian calendar, effective from 1 January 45 BCE, had a year of 365 days with an extra day every fourth year. Extra days were distributed among the months so that they totalled 365. The leap day was added to February, following the traditional intercalation point. Soon after introduction, the pontiffs misapplied the leap-year rule, adding the day every third year, and Augustus corrected this by omitting leap days for a time; the details of the correction are debated.
The month Quintilis was renamed Julius after Caesar’s death, and Sextilis later became Augustus. The Julian average year of 365.25 days was slightly longer than the true solar year, leading to the later reform of 1582, discussed under the Gregorian reform.
What the Roman calendar shows
The Roman case illustrates how a practical count can retain traces of earlier stages in its names, and how control of a calendar can become a political matter. It also shows that a twelve-month year did not arise from a single design. The number twelve emerged from the fit between the moon and the year and was reinforced by later tradition, long before it acquired any symbolic meaning of the kind discussed in the context of twelve in astrology.