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How does the Jewish calendar work?

by Rabbi Naftali Silberberg


Library » Miscellaneous » The Jewish Calendar | Subscribe | What is RSS?


The Jews follow a lunar calendar. Since the lunar month is approximately 29.5 days, the Jewish month varies, some months are 29 days and some are thirty. This adds up to an average year of 354 days. The fact that the lunar year is eleven days less than the solar year is problematic. The Torah designates Passover as the holiday of the "month of springtime"1, Shavuot is the "Harvest Holiday"2, and Sukkot is dubbed the festival of the "ingathering [of the wheat]"3.

If the Jewish calendar were to lose eleven days each year, Passover would not be restricted to springtime, every few years it would be in a different season. The same is true with all the other holidays too. For this reason, an extra month is added to the Jewish calendar every few years. On those years, called leap years, a second month of Adar is added, so we have Adar I and Adar II. This allows the lunar calendar to catch up with its solar counterpart.

The original Jewish calendar wasn't precalculated. Instead, on the 30th day of every month the Sanhedrin would be in session, awaiting witnesses who saw the crescent new-moon. If witnesses arrived, that day would be consecrated as Rosh Chodesh, thus the previous month would be a 29 day month. If no witnesses arrived, then automatically the next day was Rosh Chodesh, and the previous month was a 30 day month. Similarly, the leap years weren't precalculated. Rather, when the Sanhedrin realized that Passover would be too early on any given year, they would add a new month, thus ensuring that Passover (and all the other holidays) remained in its proper season. As soon as the Sanhedrin established Rosh Chodesh of any month, or if they decided to designate the year as a Leap Year, they would send emissaries to all the Jewish communities in the Diaspora, informing them of their decisions.

See also When and why did the Jews switch to a perpetual calendar?


  • 1. Exodus 23:15
  • 2. ibid. 23:16
  • 3. ibid.


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Torah is G–d’s teaching to man. In general terms, we refer to the Five Books of Moses as “The Torah.” But in truth, all Jewish beliefs and laws are part of the Torah.
Leap Year
Every 2-3 years an extra month is added to the Jewish calendar. Since the lunar year, which Jews follow, is 11 days shorter than the solar year, it is necessary to keep pace, so that holidays corresponding to certain seasons remain in sync. On a leap year, a second month of Adar is added.
A Biblically mandated early-spring festival celebrating the Jewish exodus from Egypt in the year 1312 BCE.
A seven day autumn festival commemorating the miracle of the Heavenly Clouds which enveloped the Jews while traveling in the desert for forty years. On this holiday we dwell in makeshift booths and shake the Four Species.
The Jewish Supreme Court. The court would convene in a designated chamber in the Holy Temple, and was comprised of 71 of the greatest scholars of the time. Continued after the destruction of the Temples, but was dissolved in the 5th century when due to Roman persecution the seat of Torah scholarship relocated from Israel to Babylon.
The twelfth month on the Jewish calendar. This month (which falls out approx. February-March), is the most joyous month on the calendar due to the holiday of Purim which is on the 14th and 15th of this month.
Rosh Chodesh
The "Head of the Month," Rosh Chodesh is observed the first day of every Jewish month. If the previous month had 30 days, then the last day of the previous month is also observed; hence a two-day Rosh Chodesh. Rosh Chodesh is a semi-holiday, marked by Torah-reading and special prayers.
Early summer festival marking the day when the Jews received the Torah at Mount Sinai in the year 2448 (1312 BCE).