Explanation of the Roman Calendar When Julius Caesar defeated Pompey at the battle of Pharsalus in 48 BC, the traditional calendar of the Roman state was in chaos, and in 45 BC, he introduced a new calendar, the Julian. Although he altered the length of the months, methods of indicating time were preserved from the ancient calendar. An understanding of both is therefore necessary.
The traditional Roman year, which may have been instituted c. 450 BC, had twelve months, and was in theory, lunar. It had four months of 31 days (March, May, July and October), seven months of 29 days (January, April, June, August, September, November and December) and one month of 28 days (February). In order to bring the calendar into line with the phases of the moon, an extra or "intercalary" month of 22 or 23 days+ the remaining five days of February was theoretically inserted every other year after the feast of the Terminalia on February 23. The system was in perpetual chaos as intercalations were not always made as they were supposed to be.
Within the months of the traditional calendar there were three days of special importance: the Calends, or first day of the month, the Nones (the fifth day in a month of 28 or 29 days, the seventh day of a month of 31 days) and the Ides (the thirteenth day of a month of 28 or 29 days, the fifteenth of a month of 31 days). To indicate a date the Romans counted backwards inclusively from the next significant day of the month. Thus December 29 was "the day before the Calends of January," January 2 was the day + 4 before the Nones of January, March 13 was the day + two before the Ides of March (for convenience I simply translate these as being three days before the Ides of March etc.= March 13, March 14, March 15).
Julius Caesar instituted a year of 3651/2 days with months of the current length and a leap year in February. But the way of indicating days within the month remained the same.