Ancient calendars were based on the lunar month and the solar year. Because the solar year is not an integral multiple of the lunar month, the months tended to drift through the seasons, and before anyone quite realized it February was the first month of the year instead of March. It was embarrassing to have a spring festival in the middle of a snowstorm, so one of the functions of the high priest was to occasionally insert or intercalate an extra month into a year to bring the seasons back into synchronization.
"Unaccustomed As I, Unh ...,"
Word Ways: Vol. 12
, Article 10.
Available at: http://digitalcommons.butler.edu/wordways/vol12/iss2/10