The Jewish calendar is significantly different from the standard Western calendar. This is primarily because the Jewish calendar is lunar (based on the cycles of the moon) whereas the Gregorian calendar is solar (based on the cycles of the sun).
A lunar year has 12 months containing 29 or 30 days each, which yields a 354-day year. To ensure the Jewish festivals always fall in the same season each year, an additional month (Adar II) is added seven times every 19 years to make up the difference.
Determining what marks the Jewish new year is a bit complicated. Based on Exodus 12:2, Nisan is considered the first month of the year. However, the year number changes on Rosh Hashanah, which is on the first of Tishrei, the seventh month of the religious year. Then there is Tu B'Shvat (15 Shvat), which is the new year "for trees."
The best way to understand this is that the Jewish calendar simply has different "years" for different purposes, just as the secular world recognizes as fiscal year, a school year and a calendar year. The bottom line: Nisan is the first month on the Jewish calendar and Rosh Hashanah (on 1 Tishrei) is the "Jewish New Year."
The year number on a Jewish calendar is based on a traditional date of creation, based on adding up the geneaologies in the Tanakh. The Jewish year 5764 began on September 27, 2003.
For convenience, many Jews use the Christian dating of years, but observant Jews make sure to use the designation CE (Common or Christian Era) instead of AD (anno domini, "in the year of our Lord"). To use the latter expression, even in abbreviation, would be to falsely (and blasphemously) imply faith in Jesus as Lord.
Each Jewish month begins with the new moon, which is called the Rosh Khodesh (Head of the Month). Rosh Khodesh was a major holiday in the First Temple period, celebrated with special sacrifices and feasts, but it was downgraded to a minor holiday after the Babylonian exile and not generally recognized today.1
The Jewish day begins and ends at sunset. Thus the Sabbath begins not at midnight Saturday morning but on Friday at sundown and the first Hanukkah candle is lit on the night of 24 Kislev.
The months of the Jewish calendar are as follows:
|Number of Days||Gregorian Equivalent|
|Kheshvan||29 or 30||October-November|
|Kislev||29 or 30||November-December|
|Adar||29 (30 in a leap year)||February-March|
|Adar II||(29 in a leap year)||March-April|
However, it has recently been reclaimed by Jewish feminists as a special day for women (Robinson, Essential Judaism, 78). ↩