:
The Hebrew or Jewish calendar is the annual calendar used in Judaism. It determines the dates of the Jewish holidays, the appropriate Torah portions for public reading, Yahrzeits (the date to commemorate the death of a relative), and the specific daily Psalms which some customarily read.
Two major forms of the calendar have been used: an observational form used prior to the destruction of the Second Temple in 70 CE, and based on witnesses observing the phase of the moon, and a rule-based form first fully described by Maimonides in 1178 CE, which was adopted over a transition period between 70 and 1178.
The "modern" form is a fixed arithmetic lunisolar calendar. Because of the roughly 11 day difference between twelve lunar months and one solar year, the calendar repeats in a Metonic 19-year cycle of 235 lunar months, with an extra lunar month added once every two or three years, for a total of 7 times per 19 years.
So in practical terms, 7 times every 19 years we have a Jewish leap year, where we add in an extra month (Adar I and Adar II instead of just Adar). Adar is the month of one of our festivals (Purim), so in a leap year, this is always celebrated in Adar II