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The Jewish calendar is a lunisolar calendar. Hence, in regular intervals a thirteenth lunar month has to be inserted in order to keep the lunar calendar aligned with the solar year. Until the Mishnaic period (10−220 CE) the calendar was based on observations with an additional month added whenever observation of agriculture-related events deemed this necessary. A new month started when the lunar crescent could be observed for the first time after new moon. Consequently, a new day began at the end of civil twilight. Starting with the Amoraic period (220−500 CE), this system was gradually replaced by a calendar scheme that is still used today. Maimonides (12th century CE) in his work Mishneh Torah gives all necessary rules for the computation of the scheme. He introduced the counting of the years starting with creation called Anno Mundi (AM). The epoch AM 1, Tishri 1 equals October 7th, 3761 BCE of the Julian calendar. For synchronisation with the solar year, the 19-year Metonic cycle is used. Leap years occur in years 3, 6, 8, 11, 14, 17 and 19 of a cycle. The Jewish month has either 29 or 30 days, and the year consists of 12 or 13 months. Both types of years - called common and embolismic years - can vary in three ways due to the additional rule, that a new Jewish year cannot start on wednesday, friday or saturday. Hence, a Jewish common year may contain 353, 354 or 355 days, and an embolismic year 383, 384 or 385 days.
