By Roman custom, February 24 is doubled in a leap year, very much like February 29 now. This intercalary month was supposedly based on observations to ensure the best possible correspondence with the seasons.
Unfortunately the Pontifex Maximus, who would normally be an active politician, often manipulated the decision to allow friends to stay in office longer or force enemies out early. Such unpredictable intercalation meant that dates following the month of Februarius could not be known in advance, and Roman citizens living outside Rome often did not know the current date.
365 Holidays a Year 