This year we celebrate two months of Adar, Adar Rishon (First Adar) and Adar Sheni (Second Adar). Why is this?

The year in the Jewish calendar consists of twelve lunar months, but the festivals follow the solar year, since several of them (Passover, Shavuot and Sukkot) must take place in certain seasons, and the seasons are determined by the earth annual revolution around the sun. Since the lunar years is roughly 354.3 days in length, while the solar year is roughly 365.5, or roughly eleven days longer, the festival would eventually fall in the wrong seasons if their occurrence followed the cycle of lunar months.

For example, Passover would be celebrated eleven days earlier in each succeeding year, and as a result would eventually be celebrated in the winter, and then in the autumn and summer, in violation of the biblical prescription that Passover must take in the month of Aviv, i.e., in the spring (Deut. 16:1). Similarly, Shavuot must occur at the time of the early harvest and Sukkot in the fall.

To prevent this difficulty, the lunar calendar is regularly adjusted to keep it in conformity with the solar year. This is done through the periodic additional, or intercalculation, of a thirteenth month, known as Adar Sheni (Second Adar), immediately after the normal month of Adar, which in leap years is known as Adar Rishon (First Adar). Since the discrepancy between the solar and lunar years amounts 207 every 19 years, the "leap month" of Adar Sheni is added to the third, sixth, eight, eleventh, fourteenth, seventeenth and nineteenth year of every 19-year period, i.e., seven times in a 19-year lunar cycle.

During such years, such as this one, most observances normally held in Adar are moved to Adar Sheni, including Purim. When a death occurs in Adar in an ordinary year, the yahrzheit (anniversary of the death) is observed in the first month of Adar, even in a leap year. Adar is 29 days long in an ordinary year, 30 days long in a leap year. Adar Sheni always has 29 days.

If you're eager to learn about Adar Sheni, when joy is said to increase, have a peek.



The Jewish calendar: How it is calculated and when it was established
The Jewish calendar: Solar/lunar structure
The zodiac in Jewish tradition




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