The biblical calendar

Nowadays, the Jewish year begins with the feast of Rosh Hashanah (Feast of trumpets, ראש השנה), which usually somewhere in September. This is not, however, the beginning of the Hebrew year as ordained by God.1
In biblical times, the year began with the month of Nisan2 (נִיסָן‎), called Aviv in the Torah (5 books of Moses: Lev. 23:5; Num. 9:1-5, 28:16). The name Nisan is of Babylonian origin. Nisan usually falls in March–April on the Gregorian calendar.

This month shall be for you the beginning of months. It shall be the first month of the year for you.
Exodus 12:2

It is during this first month that the Passover (Pesach, פֶּסַח) is celebrated.

Overview over the year

In some years, Rosh Hashanah is postponed. In that case, Kislev can be shortened to 29 days, and the year is called a short (chaser) year, or Marcheshvan can be lengthened to 30 days, and the year is called a full (maleh) year. In a regular (kesidran) year, Marcheshvan has 29 days and Kislev has 30 days though.

The calendar rules have been designed to ensure that Rosh Hashanah does not fall on a Sunday, Wednesday or Friday. This is to ensure that Yom Kippur does not directly precede or follow Shabbat, which would create practical difficulties, and that Hoshana Rabbah is not on a Shabbat, in which case certain ceremonies would be lost for a year.
The solar year is about eleven days longer than twelve lunar months. The Bible does not directly mention the addition of “embolismic” or intercalary months. However, without the insertion of embolismic months, Jewish festivals would gradually shift outside of the seasons required by the Torah. This has been ruled as implying a requirement for the insertion of embolismic months to reconcile the lunar cycles to the seasons, which are integral to solar yearly cycles.3

The following table gives an overview over the Hebrew calendar:

# Hebrew Common Length Babylonian analog Holidays Notes
1 נִיסָן Nissan 30 days Nisanu Passover Called Abib (Ex. 13:4, 23:15, 34:18; Deut. 16:1) and Nisan (Est. 3:7)
2 אִיָּר / אייר Iyar 29 days Ayaru Pesach Sheni, Lag B’Omer Called Ziv in 1 Kings 6:1, 6:37
3 סִיוָן / סיוון Siwan 30 days Simanu Shavuot
4 תַּמּוּז Tamuz 29 days Dumuzu 17th of Tammuz Named for the Babylonian god Dumuzi/Tammuz
5 אָב Ab 30 days Abu Tisha B’Av, Tu B’Av
6 אֱלוּל Elul 29 days Ululu
7 תִּשׁרִי Tishrei 30 days Tashritu Rosh Hashanah, Yom Kippur, Sukkot, Shmini Atzeret, Simchat Torah Called Ethanim in 1 Kings 8:2, 1st month of civil year.
8 מַרְחֶשְׁוָן / מרחשוון Marcheshvan, Cheshvan 29 or 30 days Arakhsamna Called Bul in 1 Kings 6:38
9 כִּסְלֵו / כסליו Kislev, Chisleu, Chislev 29 or 30 days Kislimu Hanukkah
10 טֵבֵת Tebeth 29 days Tebetu 10th of Tevet
11 שְׁבָט Shevat, Shebat, Sebat 30 days Shabatu Tu Bishvat
12L* אֲדָר א׳ Adar I* 30 days *Only in Leap years.
12 אֲדָר / אֲדָר ב׳ Adar, Adar II 29 days Adaru Purim

References

1 Article in the Jewish Encyclopedia
2 Article in the Jewish Encyclopedia
3 Article on Wikipedia

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