The Hebrew calendar or the Jewish calendar is the calendar used by the Jews to set dates of religious significance such as the Jewish holidays the date of the celebration of Bar Mitzva the annual date of commemoration of the Jewish departures etc.
The Hebrew calendar is based on the cycles of the sun and the moon where the length of the year on average is approximately 365.25 along the path of the earth around the sun and the length of the month on average is along the path of the moon around the earth i.e. approximately 29.5. Because the lunar year is missing from the solar year by about eleven days the shortfall between these two cycles is adjusted by adding a full month to the year if the calendar goes back by 30 days compared to the passage of seasons in some years.
In the contemporary Hebrew calendar whose design dates back to the year 358 AD the length of months and years is determined by an algorithm rather than by astronomical surveys.

Adapted from:
https://ar.wikipedia.org/wiki/تقويم_عبري
Translated by Google