We get many questions about the Jewish Calendar, so I thought I’d share a little overview of it.
First of all, it is important to know that the Jewish calendar uses both the sun and the moon to mark time. The sun measures a day. We find in Genesis 1:2 that G-d created light and it was good. It also says that He divided the light from the darkness and that He called the light “day” and the darkness “night.” Then it says, “And the evening and the morning were the first day.”
G-d marks the months of the year by the moon. Each new moon begins a new month, “Rosh Chodesh.” Therefore, each Biblical month contains 29 or 30 days, corresponding to a lunar month average of 29 1/2 days. We find many places in the Bible which speak of the New Moon festivals and the New Moon sacrifices. So these are appointed times which are important to the L-rd.
The problem with strictly lunar calendars is that there are approximately 12.4 lunar months in every solar year, so a 12-month lunar calendar is about 11 days shorter than a solar year and a 13-month lunar is about 19 longer than a solar year. The months drift around the seasons on such a calendar: on a 12-month lunar calendar, the month of Nissan, which is supposed to occur in the Spring, would occur 11 days earlier in the season each year, eventually occurring in the Winter, the Fall, the Summer, and then the Spring again. On a 13-month lunar calendar, the same thing would happen in the other direction, and faster.
To compensate for this drift, the Jewish calendar uses a 12-month lunar calendar with an extra month occasionally added. The extra month added is called Adar Aleph. The “permanent” month is the Adar Bet. Therefore, if someone’s birthday is in Adar, during a leap year they will celebrate it during Adar Bet.
This year is a leap year, and we are currently in the month of Adar. The holiday of Purim is the 14th of Adar. Since Adar Aleph is the added month, we will not celebrate Purim until the 14th of Adar Bet this year, which corresponds to March 24th, instead of the 14th of Adar Aleph which is February 23.