The Hebrew Calendar ~ A Luni-Solar Calendar
The Jewish calendar is based on the cycles of the sun and moon, known as a luni-solar calendar. The lunar year, which is based on the cycles of the moon, is 354 days. The solar year is 365 days, which makes the lunar year about 11 days shorter than the solar year, so there are approximately 12.4 lunar months in every solar year. To compensate for this, and to align with the solar year, an extra month is added to the Hebrew calendar - Adar I (which occurs before the regular month of Adar, which becomes known as Adar II). There is a 19 year cycle of adding the extra month. The extra month is added to the Jewish calendar in the years 3, 6, 8, 11, 14, and 17. The most recent year that the extra month was added was in Hebrew year 5776 (2016 in the civil calendar).