The Jewish Calendar is a lunar calendar, while our secular Gregorian calendar is a solar calendar. As we know, the solar calendar year is 365 days and a quarter. Because we can’t account for that bit of time that isn’t a full 24 hours in each year, every four years we add a day to the month of February, and we call it a leap year.
Well, the Jewish calendar has leap years too! But instead of an extra day, the Jewish calendar gets a whole extra month!
The Jewish lunar year is only 354 days long. If we didn’t adjust for the difference between the lunar and solar calendars, our holidays would drift around, and land in completely different seasons. The Islamic calendar is a lunar calendar that does not correct for the difference in time, and as such, their celebrations and holy days migrate greatly over time.
In the Jewish religion, many of our holidays have agricultural influences, and wouldn’t work outside of their appointed seasons. For example, Passover is also called Chag HaAviv, the holiday of the spring, and it would be odd to be celebrating a harvest holiday like Sukkot when it’s not harvest time. So, it becomes an imperative that we correct for the drift.
This year in 5779, we observe a leap year. We are currently in the month of Adar I, and next week, we begin the month of Adar II.