HMS-1 The Jewish Calendar

Halacha L'Moshe Mi Sinai
February 12, 2003 - Number 1


Judaism uses a lunar calendar for all holidays except for Shabbat which falls every seventh sunset. The calendar, however, is not a true lunar calendar. A true lunar calendar will lose 11 days against the solar year. Jewish Holidays, however, are sensitive to the seasons. Passover must fall in the spring, Sukkot must arrive in the fall, at the end of the harvest in Israel. The lunar calendar, therefore is modified to keep it in line with the solar seasons.

A lunar year has 354 days. A Jewish year can have 353, 354 or 355 days. A Jewish leap year has 383, 384 or 385 days. A month of 30 days is added to the calendar seven times in a 19 year cycle. The extra month is just before the Hebrew month of Nisan, the month where Passover falls. It is added here to make sure that the rainy season in Israel is over and the roads are dry enough so that all who wish to make a pilgrimage to Jerusalem will be able to do so without difficulty or danger.

A lunar month has 29 and 2 days. A month on the Jewish calendar will have either 29 or 30 days (you can=t have a half day). The months alternate between 29 and 30 days except sometimes Heshvan, the month after Sukkot, which should have 30 days is given 29 or sometimes the following month, Kislev, which should have 29 days, is given 30. These days are usually added or subtracted to make sure that certain holidays can not fall on the wrong day of the week. For example, Yom Kippur is on the 10th of Tishre, but it can not fall on a Friday or a Sunday. One can not be fasting and preparing for Shabbat and one can not observe Shabbat and then begin a fast. It would be too difficult for people to endure. Hoshana Rabba, the final intermediate day of Sukkot, requires that we beat willow branches in the synagogue. This is forbidden on Shabbat so Hoshana Rabbi can not fall on Shabbat. Heshvan and Kislev are therefore adjusted so that these days are not in conflict.

This year, 5763 is a leap year and we are in the middle of Adar 1. It will be followed by Adar 2 when we will celebrate Purim.
The quickest way to keep track of the Jewish Calendar is to pick one up at the synagogue just before the High Holy Days. Jewish Calendars are also available on the web and in a format that is compatible with a PDA using the Palm Platform