A lunar calendar of 12 months (consisting of 29 or 30 days each) results in a year that is shorter than a true solar year. Various methods have been used to align the two. This can be done by adding extra days or by periodically inserting an additional month, perhaps before the start of the next year. Such a system coordinates the calendar with the seasons when crops are planted or harvested.

“The rule according to which it was decided whether to intercalate or not was very simple,” says scholar Emil Schürer, in his book The History of the Jewish People in the Age of Jesus Christ, (175 B.C.–A.D. 135). “The feast of Passover, to be celebrated at full moon in the month of Nisan (14 Nisan), must always fall after the vernal [or, spring] equinox . . . If, therefore, it was noticed towards the end of the year that Passover would fall before the vernal equinox, the intercalation of a [13th] month before Nisan was decreed.”