A year of 13 months instead of 12

A Jewish "leap year" occurs seven times in a 19-year cycle and it has 13 months instead of 12. The reason for that lies in the fact that the lunar-based Jewish year has a total of 354 days, making it 11 days shorter than the 365.25 days of the solar year. To readjust both cycles, an extra month which is called Adar I (Adar Aleph), is added before the month of Adar II (Adar Bet) in a leap year.

Photo: Mosaic floor from a 6th century synagogue at Beth Alpha, Israel - showing the Hebrew calendar and the signs of the zodiac.