Metonic cycle
period of very close to 19 years that is nearly a common multiple of the solar year and the synodic (lunar) month / From Wikipedia, the free encyclopedia
The Metonic cycle is a period of about 19 years used in astronomy and calendar studies. In particular, it is often used in the structure of lunisolar calendars, because 19 solar years is almost exactly the same length of time as 235 lunar months.