Hebrew Month - Structure

Structure

The Hebrew calendar is a lunisolar calendar, meaning that months are based on lunar months, but years are based on solar years. The calendar year features twelve lunar months of twenty-nine or thirty days, with an intercalary lunar month added periodically to synchronize the twelve lunar cycles with the longer solar year. (These extra months are added seven times every nineteen years. See Leap months, below.) The beginning of each Jewish lunar month is based on the appearance of the new moon. Although originally the new lunar crescent had to be observed and certified by witnesses, the moment of the new moon is now approximated arithmetically.

Concurrently there is a weekly cycle of seven days, mirroring the seven-day period of the Book of Genesis in which the world is created. The names for the days of the week, like those in the Creation account, are simply the day number within the week, with Shabbat being the seventh day. Each day of the week runs from sunset to the following sunset and is figured locally.

The mean period of the lunar month (precisely, the synodic month) is very close to 29.5 days. Accordingly, the basic Hebrew calendar year is one of twelve lunar months alternating between 29 and 30 days: Tishrei (30 days), Marcheshvan (29 days), Kislev (30 days), Tevet (29 days), Shevat (30 days), Adar (29 days), Nisan (30 days), Iyar (29 days), Sivan (30 days), Tammuz (29 days), Av (30 days), and Elul (29 days), for a total of 354 days. In leap years (such as 5771) an additional month, Adar I (30 days) is added after Shevat, while the regular Adar is referred to as "Adar II."

There are additional rules in the Hebrew calendar to prevent certain holidays from falling on certain days of the week. (See Rosh Hashanah postponement, below.) These rules are implemented by adding an extra day to Marcheshvan (making it 30 days long) or by removing one day from Kislev (making it 29 days long). Accordingly, a common Hebrew calendar year can have a length of 353, 354 or 355 days, while a leap Hebrew calendar year can have a length of 383, 384 or 385 days.

The insertion of the leap month mentioned above is based on the requirement that Passover—the festival celebrating the Exodus from Egypt, which took place in the spring—always occur in the spring season. Since the adoption of a fixed calendar, intercalations in the Hebrew calendar have been assigned to fixed points in a 19-year cycle. Prior to this, the intercalation was determined empirically:

The year may be intercalated on three grounds: 'aviv, fruits of trees, and the equinox. On two of these grounds it should be intercalated, but not on one of them alone.

The Hebrew calendar year conventionally begins on Rosh Hashanah. However, other dates serve as the beginning of the year for different religious purposes. See New year, below.