The months of the Jewish year are lunar in nature. Unlike the months of the Gregorian solar year that is the norm in the world today, the months of the Jewish year reflect the phases of the moon. This can be seen most clearly in the length of the months. Whereas the months of the Gregorian calendar vary in length between twenty-eight and thirty-one days in order to make a solar year of 365 (or, in leap years, 366) days, the months of the Jewish year are either twenty-nine or thirty days long. This reflects the fact that a lunar month is twenty-nine and a half days in length, and the months always must begin with the new moon.
A year of twelve lunar months, however, is some eleven days shorter than a solar year. In order to ensure that the various seasonally based holidays in the Jewish calendar continue to occur at the correct season, the rabbis developed a system over time that allowed them to coordinate their lunar months with the solar year by inserting a leap month at the end of the year seven times in every 19-year cycle. This is now fixed in the third, sixth, eighth, 11th, 14th, 17th, and 19th years of the cycle. Although this is traditionally ascribed to Rabbi Hillel II in the fourth century CE, it is probable that the system in use today developed slowly during the course of the mid to late first millennium.
In order to further fine-tune their calculations, the rabbis determined that the months of Nisan (March-April), Sivan (May-June), Av (July-August), Tishrei (September-October), and Shevat (January-February) are always thirty days long. Iyyar (April-May), Tammuz (June-July), Elul (August-September), Tevet (December-January), and Adar (March-April) are always twenty-nine days long. Heshvan (October-November) and Kislev (November-December) are either twenty-nine or thirty days in length. In a leap year, there are two months of Adar, the last month of the year. When that occurs, Adar I is thirty days long, and Adar II twenty-nine. A short Jewish year, therefore, consists of 353 to 355 days, while a leap year varies between 383 and 385 days.
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