What is the Jewish Calendar?

A Quick Guide to the Hebrew Calendar

Total lunar eclipse in Jerusalem
Total lunar eclipse in Jerusalem on June 15, 2011. Uriel Sinai / Getty Images

Although the secular (also referred to as Gregorian) calendar is based on the Earth's rotation around the sun, the Hebrew calendar calculates its days, months, and years differently. 

The Basics

The Jewish calendar is lunar and is based on three things:

  • the rotation of the Earth on its axis (a day)
  • the revolution of the moon around the Earth (a month)
  • the revolution of the Earth around the sun (a year)

    The moon revolves around the Earth every 29.5 days on average, while the Earth revolves around the sun every 365.25 days, which amounts to 12.4 lunar months. 

    The Gregorian calendar abandoned lunar cycles in favor of months of 28, 30, or 31 days, but the Jewish calendar continues to hold by the lunar calendar. So, months range from 29 to 30 days to correspond to the 29.5-day lunar cycle and years are either 12 or 13 months to correspond to the 12.4-month lunar cycle. 

    Thus, the typical Jewish calendar is 12 months and 354 days long, creating an 11-day shortfall from the Gregorian calendar. 

    The Jewish Leap Year

    The Jewish calendar accommodates for the year-to-year 11-day shortfall by adding in an additional month, a policy that was enacted by Hillel II in 358 CE. The additional month falls around the Hebrew month of Adar, resulting in an Adar I and an Adar II.

    In this type of year, Adar II is always the “real” Adar, which is the one in which Purim is celebrated, yarzheits for Adar are recited, and in which someone born in Adar becomes a bar or bat mitzvah. 

    On the other hand, a special minor holiday called Purim Katan is celebrated in Adar I during these types of years. The month Adar II is also known as Adar Bet or Adar Sheni. 

    Known as a leap year or “pregnant year" (in Hebrew, a Shanah Meuberet), this phenomenon occurs seven times in a 19-year cycle during the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years.

    And, every so often, the Gregorian calendar's Leap Day, which falls on February 29th, coincides with the addition of the Jewish calendar's leap month. 

    Calculating the Jewish Year

    During the late Second Temple Period and during the time after the destruction of the Temple in 70 CE, many great sages attempted to calculate the date of creation. Most used the Torah's account of lifetimes and kingdoms to create a known date according to the Gregorian calendar.

    The calculation used today is attributed to Rabbi Yossi ben Halafta, a rabbi from the 2nd century CE, and establishes the fourth hour of Monday, October 7, 3761 BCE as the date of creation. Thus, to calculate the current Jewish year on the Gregorian calendar, just add 3760 to the Gregorian year if it is prior to Rosh HaShanah (the Jewish new year, which falls in September/October). If it is after Rosh HaShanah, add 3761 to the current Gregorian year. 

    There are, however, hundreds of different calculations that have been established over the years that vary by hundreds of years to thousands of years. Up until the reign of Queen Victoria, in fact, the date given for creation had been calculated by Bishop Usher in the 17th century as 4004 BCE.

    The Jewish Day and Month

    On the Hebrew calendar, the day begins at sundown and ends at sunset. The week itself culminates on Shabbat, which falls on Friday at sundown and ends Saturday at sunset. Even the hour in the Jewish calendar is unique and different than the typical 60-minute structure most know. 

    Each new Jewish month begins and ends with the appearance of the new moon, and the months of the Hebrew calendar are: 

    • Nisan (March-April)
    • Iyar (April-May)
    • Sivan (May-June)
    • Tammuz (June-July)
    • Av (July-August)
    • Elul (August-September)
    • Tishrei (September-October)
    • Cheshvan (October-November)
    • Kislev (November-December)
    • Tevet (December-January)
    • Sheva (January-February)
    • Adar (and, occasionally, Adar II) (February-March, sometimes April)

    Bonus Fact

    You'll notice that, throughout this article, the terms CE and BCE were used.

    Unlike in the Christian world, the Jewish world (and, notably, much of the academic world today) uses the following updated acronyms to divide time:

    • BC (Before Christ) = BCE (Before Common Era)
    • AD (Anno Domini) = CE (Common Era)