The Metonic cycle is the 19-year Hebrew-calendar cycle used to align lunar months with the solar year and help determine Rosh Hashanah dates.
A lunar year of twelve months is about eleven days shorter than a solar year. Without adjustment, festivals would drift through the seasons. The Hebrew calendar solves this by adding a leap month in seven of the nineteen years. In the traditional pattern, leap years fall in cycle positions 3, 6, 8, 11, 14, 17, and 19.
The cycle contains 235 lunar months: twelve months in each of twelve common years, plus seven extra months distributed among the leap years. This keeps Passover in spring and preserves the seasonal framework of the Jewish calendar. The Hebrew term machzor katan, meaning “small cycle,” is also used for this 19-year pattern.
The cycle does not by itself produce every Rosh Hashanah date. The calendar also uses molad calculations and postponement rules governing when the new year may begin. “Julian cycle,” “Saros cycle,” and “Indiction cycle” belong to other calendrical or astronomical systems.