The Pentecontad Calendar is a unique agricultural calendar system thought to be of Amorite origin in which the year is broken down into seven periods of fifty days ( total of 350 days ), with an annual supplement of fifteen or sixteen days. Identified and reconstructed by Hildegaard and Julius Lewy in the 1940s, the calendar's use dates back to at least the 3rd millennium BCE in western Mesopotamia and surrounding areas. Used well into the modern age, forms of it have been found in Nestorianism and among the fellaheen of modern Palestine.
In Akkadian, the pentecontad calendar was known as hamšâtum and the period of fifteen days at the end of the year was known to Babylonians as shappatum. The religious injunction to "observe the Sabbath" is thought to derive from the injunction to observe the shappatum, the period of harvest time at the end of each year in the pentecontad calendar system.
Each fifty day period was made up of seven weeks of seven days and seven Sabbaths, with an extra fiftieth day, known as the atzeret.
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