The Roman calendar

The hunter-gatherers

To the hunter-gatherers of early times, the Moon was the most convenient way of measuring the passing of time. From one New Moon to the next is 29.53 days, and this period was called a "month". Someone, usually a "priest", would be appointed to keep a look-out for the time of the New Moon: this would be the beginning of the month.

Of course, everyone was also aware of the passing of the seasons, but the start of a new year was much more difficult to measure than the start of a new month, an d the seasons were much less important in pre-agricultural times. However, it was noticed that a year lasted approximately 12 lunar months. Now, 12 lunar months is 354.37 days, not 365.24 days (which is the length of the solar year), so early lunar calendars had only 354 days, and the months alternated between 30 and 29 days:

The Babylonians

The Babylonians realised that it would be more convenient to have all the months the same length. In addition, having 12 months of 30 days would bring the length of the year up to 360 days, which was much closer to its true value:

Nabonasser

As agriculture became more important, however, it became necessary to have a proper solar year that kept in step with the seasons. This required a year of 365 days, so in 747 BC, the Babylonian king Nabonasser added an extra 5 days on to the end of the year:

The early Romans

The early Romans, on the other hand, preferred the traditional year of 354 days. The city of Rome had not yet been founded, and the people did not yet have a settled ("civilized") agricultural life-style, so a lunar calendar suited them better. However, they did like the idea of having all the months the same length, so they had 11 months of 30 days and a shorter month of 23 days, which they called "February" (meaning "Purification") because it was intended to "purify" the error created by having all the other months slightly too long. In addition, there was an extra day (or sometimes two days) at the end of the year when the priests would look out for the New Moon which would start the next year:

(with the numbers in square brackets showing that the extra day became two days approximately every three years).

Romulus

The eleven 30-day months were named after various Roman gods, and, because some gods were more important than others, their months were lengthened, while the months named after the lesser gods were shortened (all the while maintaining a year-length of 354 days), so that by the time Romulus came to found the city of Rome in 753 BC, the calendar was in complete disarray, with some months containing as few as 20 days while others contained 35 or more! So Romulus decided to scrap the entire lunar calendar and replace it with a solar one that was more in tune with the new Agricultural Age. In the new Roman calendar, the working year (i.e. spring, summer and autumn) was to consist of 10 named months, starting at the spring (vernal) equinox, with an un-named (and un-numbered) winter period, when no-one worked on the land. Each of the months had 30 days, except for those which contained one of the four traditional holidays (viz. March, May, Quintilis and October), when an extra day would be added so that the length of the "working month" would be the same. Thus, the year consisted of (10*30)+4=304 days, which meant that the winter period would tend to last 61 or 62 days:

Spring equinox: Mar. 1
Winter solstice: Dec. 1

Numa Pompilius

However, there was a backlash by those who preferred the old lunar calendar, so the next Emperor, Numa Pompilius, in 715 BC introduced a compromise: the length of a normal year would return to 354 days, but an occasional "intercalary" month, called "Mercedinus" (or "Mercedonius") would be added to bring the length of the average year up to 365 days. This was all done by adding two extra months of 28 days each: one called "January" to the beginning of the year and the other called "February" at the end of the year, while simultaneously reducing the six 30-day months to 29 days each. The month of Mercedinus had 22 days and was added every other year, not at the end of February but following the "traditional" last day of the year: the 23rd of February! This meant that the remaining five days of February would then follow the intercalary month:

Spring equinox: Mar. 1-23 (variable); Mar. 12 (average)
Winter solstice: Dec. 6-28 (variable); Dec. 17 (average)

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