The calendar of ancient Rome, like that of the Greek city-states, was essentially a lunar calendar with an extra, or intercalary, month inserted occasionally to keep the months more or less in step with the seasons. There were twelve months, and they were named, in order: Martius, Aprilis, Maia, Junius, Quintilis, Sextilis, September, October, November, December, Januarius and Februarius. Apart from Quintilis and Sextilis, these names have come down to us almost unchanged in over 2500 years. The names of Quintilis to December are based on the Latin words for "five" to "ten", and we can therefore deduce that the Roman year began with March.
The Romans were very superstitious. They regarded odd numbers as lucky and even numbers as unlucky, and so all of the months except February had an odd number of days: March, May, Quintilis and October had 31, February had 28 and the remainder had 29. This gave 355 days, roughly equal to 12 lunar months. The intercalary month was added, when needed, at the end of February, and on such occasions, February itself was shortened to 23 days.
Each month had three special days: the Kalends, the Nones and the Ides. The Kalends was the first day of the month, and this is the origin of the word "calendar" itself. The Nones was the 5th day of most months, but the 7th day of the long months (March, May, Quintilis, October). The Ides was the 13th, except for the long months, in which it was the 15th. Anyone who knows a little Shakespeare will remember that Julius Caesar was warned to beware the Ides of March - the 15th of March.
The Romans did not count the days of the month in the way that we do. Instead, they always counted towards the next of the three named days. Thus the day after the Kalends of March was not called March 2nd, but ante diem sextum Nonas Martias or "day six before the Nones of March", abbreviated to a.d. VI Non. Mar. March thus progressed like this:
|2nd||ante diem VI Nonas Martias|
|3rd||ante diem V Nonas Martias|
|4th||ante diem IV Nonas Martias|
|5th||ante diem III Nonas Martias|
|6th||pridie Nonas Martias|
The 6th, the day before the Nones, was pridie Nonas Martias, literally "the day before the Nones of March", The Nones itself was included in this countdown, which is why the 5th is called the third day before the Nones and not the second.
After the Ides, the dates were counted down towards the Kalends of the following month, so that March 16th was named ante diem XVII Kalendas Aprilis or "the 17th day before the Kalends of April", even though it was recognised as part of the month of March.
The Romans believed that certain days were more auspicious than others for carrying out important events such as business contracts, religious rites and even battles. Only the priests, led by the Pontifex Maximus, could tell a Roman citizen whether a given day was auspicious or not, and naturally they made a charge for each inquiry.
The priests also decided when intercalary months were needed, so they had complete control over virtually every aspect of public and private life through the calendar. However, they had no formal rules to tell them when intercalation was required, and in any case they were rather careless, so that by the time that Julius Caesar became Pontifex Maximus, the calendar had slipped by almost three months with respect to the seasons.
In order to bring the calendar back into line with the seasons, Caesar ordered that three intercalary months should be added at the end of the year which we know as 46 B.C. He also re-arranged the lengths of the months, giving each month its present duration.
But Caesar's most significant reform was to reject the lunar month completely and to adopt a solar year whose average length was 365.25 days. He introduced the four-year cycle of leap years which we still use today. The extra day was added at the end of the Roman year, after the last day of February.
Once again, carelessness prevailed. The priests applied the intercalation every three years, not four. Perhaps it arose from the superstition: 4 is an even number, and hence unlucky. Whatever the cause, the result was that the year 8 B.C. began three days late.
Augustus Caesar corrected the error by omitting leap-years until A.D. 8, and the Julian calendar was observed without further change until the great reform of Pope Gregory XIII in 1582.