The Jewish Calendar
by Wayne D. Turner
From BibleTrack
Copyright 2007-2008

Most often, only the numbers of the months are mentioned in the Old Testament. The months of the Jewish calendar are designated as follows:
Month Name Month Number Number of Days Gregorian Equivalent
Nisan 1 30 days March-April
Iyar 2 29 days April-May
Sivan 3 30 days May-June
Tammuz 4 29 days June-July
Av 5 30 days July-August
Elul 6 29 days August-September
Tishri 7 30 days September-October
Cheshvan 8 29 or 30 days October-November
Kislev 9 30 or 29 days November-December
Tevet 10 29 days December-January
Shevat 11 30 days January-February
Adar (Adar I in leap years, see below) 12 30 days February-March
Adar II (leap years only, see below) 13 in leap years 29 days February-March

Leap years: A second month called Adar is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of a 19-year cycle. In those leap years, Adar is called Adar I and the extra month of 29 days is called Adar II.

Explanation of the Jewish Calendar:
The Hebrew calendar is based upon lunar cycles. That being the case, the dates in the Jewish calendar do not correspond each year to the same dates in the Gregorian calendar. However, due to the corrections of both calendars, they are realigned (within a day or so) at the end of each 19-year cycle. While the Gregorian calendar has a leap year every four years (with a few exceptions), the Hebrew calendar is more complicated than that since it is based on lunar cycles; it provides for a realignment 7 times in 19 years with the addition of a 13th month 7 times during that 19-year cycle. A lunar month is sometimes 29 days and sometimes 30 days, averaging out to approximately 29.5 days per month over the period of an entire year.

Prior to the modern system of calculating the Jewish year adopted around 359 A.D., the Jewish calendar was calculated on the fly (so to speak). On the 30th day of the month, the governing council in Jerusalem would entertain withnesses to a new moon. If such witnesses came forth, a new month was declared beginning that day. If not, a new month was not declared until the day following the 30th day. The first new moon after spring was regarded as the first month of the new year. If signs of spring seemed to be appearing later than normal, an additional month was added to compensate for the deviation from the solar year. Modern astronomy provided the ability to put an exact algorithim to the calculation of the Jewish calendar.

The date for Easter each year is based upon the lunar cycle as well. Easter today is celebrated on the Sunday following the first full moon (aka Paschal Full Moon) after the spring equinox (March 21 or so). Therefore, Easter falls between March 22 and April 25 each year. The Passover is celebrated on the 14th day of the 1st month. Therefore, for rough calculation purposes, the approximate times for the Jewish festivals listed in the table above can be calculated by counting the number of months from Easter, minus a couple of weeks to back up to the appearance of the new moon that month. More specifically, back up 14 days from the Jewish celebration of the Passover to find the 1st day of the 1st month. However, when the Jewish calendar has an extra month because of leap month, Easter and Passover can be separated by several days or even weeks.