The Hebrew Calendar

The civil (or Julian) calendar is based on the cycle of the earth around the sun. The length of this cycle, the solar year, is very close to 365 1/4 days.

The Hebrew calendar on the other hand is based on the cycle of the moon around the earth. The length of this cycle, the lunar month, is about 29 days. Twelve lunar months make therefore about 354 days, which is 11 1/4 days shorter than the solar year.

In biblical times, the arrival of the new month was determined by watching the phase of the moon. However in modern times a fixed calendar is used in which the length of the months alternates between 29 and 30 days.

We also need to understand that Israel has two calendars; one, their civil calendar, begins in the fall (Tishri, about September on our calendar); the other, their religious calendar, begins in the spring (Nisan, about April on our calendar). Religious and civil dating systems were based on both lunar and solar cycles. In the calendar used by Jews today, a day is counted from sunset to sunset, a week comprises 7 days, a month has 29 or 30 days, and a year has 12 lunar months plus approximately 11 days. In order to bring the calendar in line with the annual solar cycle, a 13th month of 30 days is added in the 3rd, 6th, 8th, 11th 14th, 17th, and 19th years of a 19-year cycle. Therefore, a leap year may have from 383 to 385 days. The Jewish Era in use today was accepted around the 9th century AD and is based on Biblical calculations placing the creation in 3761 BC.

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