Intercalary month

Since the Jewish calendar was based on the lunar month (29.5 days) the lunar year (354.25 days) fell short of the solar year (365.25 days). Therefore an intercalary month was inserted every two or three years to make adjustments for the lunar calendar’s shortage of days. Although not directly stated this concept is implied in Num 9:11; 2 Chron 30:2-3, and 1 Kings 12:32-33.