Metonic cycle - Wikipedia
https://en.wikipedia.org/wiki/Metonic_cycle
Application in traditional calendars. In the Babylonian and Hebrew lunisolar calendars, the years 3, 6, 8, 11, 14, 17, and 19 are the long (13-month) years of the Metonic cycle.This cycle forms the basis of the Greek and Hebrew calendars, and is used for the computation of the date of Easter each year.. The Babylonians applied the 19-year cycle since the late sixth century BC.
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