The Hebrew Calendar

The Hebrew calendar is one of the most complex, and interesting, lunar calendar systems in use. It is not generally used for civil purposes - the Gregorian calendar serves that purpose - but it is used to determine the dates of major festival days and days of observance. Thus, the rules of the calendar are quite complex because, not only does it need to keep synchronised as nearly as possible to the lunar cycles, it also has to ensure certain dates do not occur on certain days of the week, e.g. Yom Kippur (10 Tishri) must not occur on a Friday or a Sunday.

Six types of year

In order to satisfy all the religious requirements the calendar has six different lengths of year. These are based around common years, i.e. 12 months alternating between 30 and 29 days in length, and leap years, when a 13th, 30 day, months in added. In a 19 year period there are 7 leap years. In addition a year can be regular (354 or 384 days), deficient (353 or 383 days) or complete (355 or 385 days) . The structure of each of these years is summarised below:

Month
Deficient
Regular
Complete
Nisan
30
30
30
Iyyar
29
29
29
Sivan
30
30
30
Tammuz
29
29
29
Av
30
30
30
Elul
29
29
29
Tishri
30
30
30
Heshvan
29
29
30
Kislev
29
30
30
Tevet
29
29
29
Shevat
30
30
30
{Adar I}
{30}
{30}
{30}
Adar {II}
29
29
29
Total
353 {383}
354 {384}
355 {385}

It can be seen from this table, then, that in a regular, common year the lengths of the months alternate evenly between 30 and 29 days. In a leap year, an extra month, Adar I, with 30 days, is inserted into the calendar. In those years, Adar, becomes Adar II. In a deficient year a day is deducted from Kislev and in a complete year a day is added to Heshvan.

There are seven leap years in the 19-year cycle. These occur in years 3, 6, 8, 11, 14, 17 and 19 of the cycle. The cycle at the time of writing began on 2nd October 1997 (5758) and thus the year, 5768 (2007-2008), is the 11th year of the cycle, which will be a leap year.

Further reading

See The Hebrew Calendar for further information.