Hebrew calendar (Jewish calendar)

Basic structure and characteristics

The calendar combines lunar months of 29 or 30 days with a year length that alternates between 12 and 13 months. A common (non‑leap) year has 12 months, while certain years add a whole extra month (Adar II) to realign the lunar months with the solar cycle. Days begin at sunset, and months begin with the new moon. Years are counted from a traditional epoch.

Months and leap years

• Typical month names include: Tishri, Cheshvan, Kislev, Tevet, Shevat, Adar (Adar I and Adar II in leap years), Nisan, Iyar, Sivan, Tammuz, Av, and Elul.
• Leap years occur seven times in a 19‑year Metonic cycle to keep Passover and other festivals in their proper seasons.

History and development

The calendar has ancient roots and was progressively standardized over centuries. Elements derive from Biblical practice (linking months to moon phases) and later rabbinic and astronomical refinements that produced fixed rules for intercalation and year length. Medieval and post‑medieval scholars clarified calculations to ensure long‑term stability.

Uses and cultural importance

Beyond setting festival dates, the calendar determines the timing of a Bar Mitzvah or Bat Mitzvah, the observance of a yahrzeit (the anniversary of a death), public prayers, and community rituals. Jewish communities around the world consult it for liturgical reading cycles and holiday planning.

Notable distinctions

• The day begins at sunset, not midnight, affecting observance windows.
• Religious and civil calendars may differ in epoch or practice; local communities sometimes follow minor variants in calculation.
• Traditional counting and modern astronomical approaches coexist in practice.

For further reading on chronology and practice, see overviews and community guides: authoritative discussions and reference pages provide practical calendars and explanations of rules used today.