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Jewish Calendar
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The date of Jewish holidays does not change from year to year. Jewish holidays are celebrated on the same day of the Jewish calendar every year, but the lunar year of the Jewish calendar is not the same length as the solar year of the Gregorian calendar used by most of the western world, so the date shifts on the Gregorian calendar.

Background and History

The Jewish calendar is based on the lunar year, with each month beginning on the new moon. Rosh Chodesh, the first of the month, begins when the first sliver of moon becomes visible after the dark of the moon.

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 loses about 11 days every year and a 13-month lunar gains about 19 days every year. The months on such a calendar "drift" relative to the solar year. On a 12 month calendar, the month of Nissan, which is supposed to occur in the Spring, occurs 11 days earlier each year, eventually occurring in the Winter, the Fall, the Summer, and then the Spring again. To compensate for this drift, an extra month was occasionally added: a second month of Adar. The month of Nissan would occur 11 days earlier for two or three years, and then would jump forward 29 or 30 days, balancing out the drift.

In the fourth century, Hillel II established a fixed calendar based on mathematical and astronomical calculations. This calendar, still in use, standardized the length of months and the addition of months over the course of a 19 year cycle, so that the lunar calendar realigns with the solar years. Adar II is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle. The new year that began Monday, September 25, 1995 (Jewish calendar year 5756) was the 18th year of the cycle. Jewish year 5758 (beginning October 2, 1997) was be the first year of the next cycle.

In addition, Yom Kippur should not fall adjacent to a Shabbat, because this would cause difficulties in coordinating the fast with the Shabbat, and Hoshanah Rabba should not fall on Shabbat because it would interfere with the holiday's observances. A day is added to the month of Heshvan or subtracted from the month of Kislev of the previous year to prevent these things from happening.

Numbering of Jewish Years

The year number on the Jewish calendar represents the number of years since creation, as calculated by adding up the ages of people in the Bible back to the time of creation.

The "first month" of the Jewish calendar is the month of Nissan, in the spring, when Passover occurs. However, the Jewish New Year is in Tishri, the seventh month, and that is when the year number is increased. The Jewish calendar has different starting points for different purposes.

Months of the Jewish Year

Month Length Gregorian Equivalent
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
Heshvan 29 or 30 days October-November
Kislev 30 or 29 days November-December
Tevet 29 days December-January
Shevat 30 days January-February
Adar 29 or 30 days February-March
Adar II 29 days March-April

In leap years, Adar has 30 days. In non-leap years, Adar has 29 days.

The length of Heshvan and Kislev are determined by complex calculations involving the time of day of the full moon of the following year's Tishri and the day of the week that Tishri would occur in the following year.

Note that the number of days between Nissan and Tishri is always the same. Because of this, the time from the first major festival (Passover in Nissan) to the last major festival (Sukkot in Tishri) is always the same.

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