The pre-Julian calendar was lunar with a total of 355 days. To keep the months in line with the seasons it had been the practice to add (intercalate) an extra month-during February every few years, but the Romans, like the Greeks, never did this with regularity. Each month was divided into three parts by means of special days called the Kalends, Nones, and Ides. Since the frame of reference was always one of looking forward to the arrival of these days, each of the other days was given a number reflecting how many days had to pass before they did arrive. The Kalends were always the first day of each month, the Nones were either the seventh day (of March, May, July, and October) or the fifth (of all the rest), while the Ides were either the fifteenth day (of March, May, July, and October) or the thirteenth (of all the rest). The Roman method of counting these days was inclusive. Thus, e.g., the Roman date ‘on the third day before the Ides of January’ is by our method of counting actually the second day before the Ides of that month (January 11).

This calendar was changed by Julius Caesar in 45 BC to one based on the sun, following the Egyptian model, but with proper intercalation. Thereafter the number of days in the months were: September, April, June, and November had 30 days; February had 28; all the rest had 31.