The Hebrew Calendar System

A lunisolar calendar explained for English speakers

The Hebrew (Jewish) calendar is a lunisolar calendar — it tracks both the phases of the Moon and the position of the Sun. Months follow the lunar cycle (new moon to new moon), while the year stays aligned with the solar seasons through the insertion of a leap month seven times every nineteen years.

Each month has either 29 or 30 days, and a normal year has 12 months (about 354 days). Because this is roughly 11 days shorter than the solar year of 365 days, a leap month — Adar Sheni (אדר ב׳) — is added in years 3, 6, 8, 11, 14, 17, and 19 of a 19-year cycle, the Metonic cycle.

How the months work

A Hebrew month begins near the astronomical new moon and lasts either 29 or 30 days. The months are: Nisan (ניסן), Iyar (אייר), Sivan (סיון), Tammuz (תמוז), Av (אב), Elul (אלול), Tishrei (תשרי), Marcheshvan (מרחשוון), Kislev (כסלו), Tevet (טבת), Shevat (שבט), and Adar (אדר). In a leap year a second Adar is added before Nisan so that the springtime festival of Pesach stays in the spring.

The four rules of postponement (Dechiyot)

The Hebrew calendar uses four arithmetic rules called dechiyot (דחיות) to postpone the start of a new year (Rosh Hashanah) by a day, to prevent certain religious observances from falling on inconvenient days of the week.

The 19-year cycle (Metonic)

Seven leap years in nineteen keep the lunar months aligned with the solar seasons — the same relationship the Chinese calendar also uses. If you were born in 5784, your Hebrew birthday will not recur on the same Gregorian date for another 19 years.