The Jewish Calendar
The ancient Jewish Calendar had a year consisting of 12 or 13 months as decided by the authorities, each month beginning when the crescent Moon had actually been observed. It was replaced by the present fixed calendar in the 4th century AD. This is based on the Metonic cycle of 19 solar years which very nearly equal 235 lunations. In each cycle numbers 3, 6, 8, 11, 14, 17, and 19 are 'embolismic' years having 13 months, while the other 12 are 'common' years having 12 months. The 12 months of the regular common year contain alternately 30 and 29 days; the extra month, First Adar, which in embolismic years is inserted between the fifth month (Shevat) and sixth month (Adar, or Second Adar as it is called in leap years) of the common year, has 30 days. Because of restrictions as to the days of the week on which certain religious fasts or festivals can fall, the year cannot begin on a Sunday, Wednesday, or Friday. To avoid this happening the length of the year is altered by a day, so that a year can be deficient, regular, or abundant according to whether it has 353, 354, 355, 383, 384, or 385 days in it. The extra day is either added to the second or subtracted from the third month according to whether the year is abundant or deficient. The years are reckoned from the era of the Creation, which is taken to be 7 October 3761 BC. The Jewish year AM 5738 began on 13 September 1977. The Jewish day begins nominally at sunset but actually at 6 p.m.; its length is 24 hours but each hour is divided into 1080 halaquim. The New Year, Rosh Hashanah or 1 Tishri, falls between 5 September and 5 October, and is followed on 10 Tishri by Yom Kippur, the Day of Atonement.