Jewish Calendar

By Mary Jane Chaignot

Categories: Early Christianity


I'm interested in knowing more about the Jewish calendar. How are festival days chosen? Does it correspond to the Gregorian calendar?


Festival days are not "chosen." The dates remain the same year after year. For example, Hanukkah starts on the 25th of Kislev every year. The problem is that the 25th of Kislev might come at a different time every year according to the Gregorian calendar, which is the one used by most of the western world. The reason for this is that the Jewish calendar is based on three astrological events. A "day" is based on one complete revolution of the earth on its axis. A "month" is based on one revolution of the moon around the earth. A "year" is based on one revolution of the earth around the sun. The moon makes one revolution roughly every 29 ½ days. The earth moves around the sun approximately once every 365 ¼ days.

In contrast, the Gregorian calendar is not based on the lunar cycle and simply determines the length of each month to be 30-31 days (excluding February, which can be either 28 or 29 days).

Jewish months, then, are either 29 or 30 days – never 31. And the Jewish year is either 12 or 13 months. That makes the Jewish year either 11 days shorter or 19 days longer than a solar year based on the Gregorian calendar. To keep things in sync, they use a 12-month lunar calendar for a few years, and then skip to the 13-month to keep it in balance. That way, for example, Passover, which occurs in the spring, always occurs in the spring. Otherwise, it would occur 11 days earlier every year, until Passover would be in the winter or fall.

This calendar has been established since the time of Hillel II (fourth century CE). It was based on mathematical and astronomical calculations. It fixed the calendar over a 19 year cycle. So every 19 years, the Jewish calendar matches the Gregorian one. The extra month (Adar I) is added in the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years of a 19-year cycle. Interestingly, we are now in the 11th year of that cycle, which began in Oct 1997.

According to the Jewish calendar, this is the year 5769. It supposedly dates back to the time of creation, though many Jews (even orthodox) do not insist that the earth has only been around for 5700 years. The marking of time in the Genesis stories do not have to correlate with a 24-hour day.

The first month of the Jewish calendar is Nissan, which comes in the spring. It is the month of Passover. That, however, is not the Jewish New Year, which is Tishri (the seventh month). So the Jewish New Year begins in the month of September, according to the Gregorian calendar. The names of the months date back to the time of Ezra and reflect Babylonian month names:

  • Nissan, 30 days, March-April
  • Iyar, 29 days, April-May
  • Sivan, 30 days, May-June
  • Tammuz, 29 days, June-July
  • Av, 30 days, July-August
  • Elul, 29 days, August-September
  • Tishri, 30 days, September-October
  • Cheshvan, 29-30 days, October-November
  • Kislev, 29-30 days, November-December
  • Tevet, 29 days, December-January
  • Shevat, 30 days, January-February
  • Adar 1, 30 days, February-March (leap years only)
  • Adar, 29 days, February-March

The Jewish day begins at sunset and goes until the following sunset. The days of the week are numbered, beginning with "Sunday" as day one and ending with the seventh day, "Sabbath." The months are, obviously, based on the phases of the moon. The month begins with the beginning of the new moon. In ancient days, this was determined by observation. Today, it results from systematic calculations. The lunar month is exactly 29 days, 12 hours, 44 minutes, and 30 seconds long.

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