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 month of Adar, known as Adar Rishon (First Adar).
Since the discrepancy between the solar and
lunar years amounts 207 every 19 years, the extra Adar 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 leap years, 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 Rishon 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 Rishon and Adar Sheni, when
joy is said to increase, have a peek.