The Hebrew/Jewish Calendar
The Hebrew calendar (Hebrew: ha'luach ha'ivri) or Jewish calendar is a lunisolar calendar used by Jews and the followers of Judaism, now predominantly for religious purposes. It is used to reckon the Jewish New Year and dates for Jewish holidays, and also to determine appropriate public reading of Torah portions, Yahrzeits (dates to commemorate the death of a relative), and daily Psalm reading, among many ceremonial uses. Originally the Hebrew calendar was used by Jews for all daily purposes. During the era of the Roman occupation (1st century BCE), Jews began additionally following the imperial civil calendar for civic matters such as the payment of taxes and dealings with government officials.
The principles of the Hebrew calendar are found in the Torah, which contains several calendar-related commandments, including God's commandment during the Exodus from Egypt to fix the month of Aviv as the first month of the year. The Babylonian exile in the 6th century BCE influenced the calendar, including the adoption of Babylonian names for the months.
During the Tannaitic period, the Hebrew calendar was observational, with the beginning of each month determined by the high court based on the testimony of witnesses who had observed a new crescent moon. Through the Amoraic period and into the Geonic period, the purely empirical calendar was displaced by calendrical rules, which finally became systematically arranged into a computed calendar. The principles and rules of the current calendar are fully described by Maimonides in the Mishneh Torah.
Because of the roughly eleven-day difference between twelve lunar months and one solar year, the year lengths of the Hebrew calendar vary in a repeating 19-year Metonic cycle of 235 lunar months, with an intercalary lunar month added according to defined rules every two or three years, for a total of 7 times per 19 years. Seasonal references in the Hebrew calendar reflect its development in the region east of the Mediterranean Sea and the times and climate of the Northern Hemisphere. The Hebrew calendar's year is longer by about 6 minutes and 25+25/57 seconds than the present-day mean solar year, so that every 224 years, the Hebrew calendar will fall a full day behind the modern fixed solar year, and about every 231 years it will fall a full day behind the Gregorian calendar year.
[From Wikipedia, the Free Encyclopedia as made available under the terms of the GNU Free Documentation License.]