Jewish Calendar

Hillel II instituted a permanent calendar based on mathematical and astronomical calculations in 358, removing the necessity for eyewitness observations of the new moon, which marks the start of a new month.

Due to uncertainty about the timing of the new moon, ancient law mandated that people living outside of Israel celebrate two days instead of one for each holiday, with the exception of Yom Kippur, the Day of Atonement.

This tradition had to be kept even after the calendar was created, according to the Talmud.

The Jewish period starts with the creation of the world, which is generally dated at 3761 B.C. Hillel's calendar has remained mostly unaltered since the eleventh century, with very minor changes.

A day is measured from sunset to sunset, a week is seven days long, a month is either twenty-nine or thirty days long, and a year is made up of twelve lunar months plus about eleven days, or 353, 354, or 355 days.

A thirteenth month of thirty days is intercalated in the third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth years of a nineteen-year cycle to reconcile the calendar with the yearly solar cycle; a leap year may have from 383 to 385 days.

The civil calendar starts with Tishri, whose first day is Rosh Hashanah, the Jewish New Year. On Nisan 15, Passover, the religious calendar cycle starts (Pesach).

The Babylonians provided the names for the months of the Jewish calendar.

The months are generally referred to in numerical sequence in the preexilic books of the Bible, starting with Tishri, although there are four months recorded with other names:

Nisan/Abib, Iyyar/Ziv, Tishri/Ethanim, and Heshvan/Bul: Nisan/Abib, Iyyar/Ziv, Tishri/Ethanim, and Heshvan/Bul: