The pentecontad calendar (from Koine Greek: πεντηκοστή, romanized: pentēkostē, lit. 'fiftieth') is an agricultural calendar system thought to be of Amorite origin in which the year is broken down into seven periods of fifty days (a total of 350 days), with an annual supplement of fifteen or sixteen days. Identified and reconstructed by Julius and Hildegaard 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 the Ethiopian Jews and among the Fellahin 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.

Used extensively by the various Canaanite tribes of Palestine, the calendar was also thought to have been used by the Israelites until the official adoption of a new type of solar calendar system by Solomon.

Philo expressly connected the "unequalled virtues" of the pentecontad calendar with the Pythagorean theorem, further describing the number fifty as the "perfect expression of the right-angled triangle, the supreme principle of production in the world, and the 'holiest' of numbers".

The calendar was used among Ethiopian Jews for a millennium. Till modern times Shabbat festivals are still celebrated currently by the Beta Israel. According to this tradition: "The Sabbaths are divided into cycles of seven. A special prayer, is recited at sunset and reflects the particular characteristics of each Sabbath. The seventh Sabbath, the Legatä Sänbät, is the holiest of all, and there are extra prayers, festivities and a special sanctification service."

Tawfiq Canaan (1882–1964) described the use of such a calendar among Palestinians in Palestine, as did his contemporary Gustaf Dalman, who wrote of the practices of Muslim agriculturalists who used Christian designations for the fiftieth day, "which in turn overlaid far more ancient agricultural practices: grape-watching, grape-pressing, sowing, etc."

Stephan Hanna Stephan provides details about the structure of this calendar as it was being practiced in the early 20th century in an article in the journal of the Palestine Oriental Society entitled The Division of the Year in Palestine. He notes that the first period of fifty days begins on Easter (by the Julian calendar used by the Eastern Orthodox Church) and continues until Pentecost, and the harvesting of lentils. The second period immediately begins and coincides with the harvesting of wheat and barley and ends with the feast of Mar Elias (July 20). The third period coincides with the harvesting of grapes and figs and ends on Eid al-Saleeb (September 14). The fourth period sees the harvesting and pressing olives and covers a period of exactly 50 days (like the first) extending to November 3, which marks Eid al Ludd. The fifth marks a time of ploughing and sowing and extends until Christmas. The sixth ends when the lenten fast begins and the seventh period ends with Easter again. He also mentions another concomitant system of dividing the year into periods of 40 and 50 days.

Julian Morgenstern argued that the calendar of the Book of Jubilees has ancient origins as a somewhat modified survival of the pentecontad calendar.