Hebrew calendar
A year of 12 lunar months would be 354 days long; but a Hebrew calendar year must be a solar year—about 365 days long. Also, a Hebrew calendar year must have either 12 months or 13 months. It is not allowed to have "part" of a month, so a thirteenth month is added seven times in every cycle of nineteen years. This is an adaptationn 2 of the Metonic cycle, a calendar cycle that was well known in ancient times. By conventionn 3, the Hebrew calendar adds the extra month during years 3, 6, 8, 11, 14, 17 and 19 of the cycle.