The Hebrew Calendar

The Jewish calendar is based on the lunar cycle. The beginning of the moon’s cycle appears as a thin crescent. That is the signal for the new Jewish month. The moon grows until it is full, the middle of the month, and then it begins to wane until it cannot be seen. It remains invisible for approximately two days,  then a thin crescent appears, and the cycle repeats again and again

The entire cycle takes approximately twenty-nine and a half days. Since a month needs to consist of complete days, a Jewish month is sometimes twenty-nine days long (known as chaser, “missing”), and sometimes thirty (meaning malei, “full”).

Knowing exactly when the month begins has always been important in Jewish practice, because the Torah schedules the Jewish festivals according to the days of the month.

The first day of the month, as well as the thirtieth day of a malei month, is called Rosh Chodesh, the “Head of the Month,” and has somewhat festive status.

The Jewish year starts on Rosh Hashanah, "the Head of the Year," the day when Adam and Eve were created. First day of Tishrei in the Jewish Calendar. (Note the day begins at sundown on the first of Tishrei and ends at sundown on Tishrei 2)  Tishrei coincides with September-October on the Gregorian Calendar.

A standard Jewish year has twelve months; six twenty-nine-day months, and six thirty-day months, for a total of 354 days. This is because the months follow the lunar cycle which is approximately twenty-nine and a half days. Due to variations in the Jewish calendar,s

The Jewish calendar must coordinate with the cycle of the sun and seasons which are determined by the solar orbit. A  lunar year, twelve lunar months added together only adds up to about 354 days. A solar year, at almost 365.25 days is around eleven days longer.. If no adjustment is made to the calendar Passover would occur eleven days earlier each year, eventually drifting into winter, then fall, summer, and then spring again. The solution to reconcile the Jewish Calendar to the solar cycle is to add one thirty day month ebout every third year, creating a leap year, referred to in Hebrew as "shanah meuberet" or pregnant year. The month is added to the month of Adar and in these leap years Adar is referred to as Adar I and Adar II.

To find the corresponding Jewish year for any year on the Gregorian calendar, add 3760 to the Gregorian number, if it is before Rosh Hashanah. and 3761 afterwards.

Hebrew and Gregorian Calendars