The pentecontad calendar (from πεντηκοντάς pentēkontás) 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 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.
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 King Solomon.
The liturgical calendar of the Essenes at Qumran was a pentecontad calendar, marked by festivals on the last day of each fifty-day period such as the Feast of New Wine, the Feast of Oil, and the Feast of New Wheat.
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".
Tawfiq Canaan (1882–1964) described the use of such a calendar among Palestinians in southern 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."
- Roger Thomas Beckwith (2005). Calendar, Chronology and Worship: Studies in Ancient Judaism and Early Christianity. BRILL. p. 26. ISBN 90-04-12526-4.
- Hebrew Union College (1924). Hebrew Union College Annual. p. 75.
- Lance Latham (1998). Standard C Date/Time Library: Programming the World's Calendars and Clocks. Focal Press. p. 37. ISBN 0-87930-496-0.
- Pi Gamma Mu (1981). Social Sciences. p. 25.
- Eviatar Zerubavel (1989). The Seven Day Circle: The History and Meaning of the Week. University of Chicago Press. p. 8. ISBN 0-226-98165-7.
- Julian Morgenstern (1966). The Rites of Birth, Marriage, Death, and Kindred Occasions Among the Semites. Hebrew Union College Press. p. 282.
- Geza Vermes (1995). The Dead Sea Scrolls in English. Continuum International Publishing Group. p. 54. ISBN 1-85075-563-9.
- André Dupont-Sommer (1956). The Jewish Sect of Qumran and the Essenes: New Studies on the Dead Sea Scrolls. Macmillan. p. 1.
- Joan E. Taylor (2003). Jewish Women Philosophers of First Century Alexandria. Oxford University Press. ISBN 0-19-925961-5.
- Millar Burrows (1955). The Dead Sea Scrolls. Viking Press. p. 241.
- Jonhatan Ben-Dov, The_History_of_Pentecontad_Time_Periods (I), in: A Teacher for All Generations. Essays in Honor of James C. VanderKam, (Gen. ed. E. Mason; JSJSup 153; Leiden: Brill, 2011), vol. I, pp. 93–111. This paper rebuts most of previous theories presented above.