Hebrew Calendar 101: Months, Holidays, and How It Works

Hebrew Calendar 101: Months, Holidays, and How It WorksThe Hebrew calendar (also called the Jewish calendar) is a lunisolar system used primarily for Jewish religious observance and cultural life. It coordinates lunar months with the solar year so festivals fall in their appropriate seasons. This article explains its structure, how months and years are determined, major holidays, conversion basics, and practical uses.

Overview: Lunisolar design

• The Hebrew calendar is lunisolar: months follow the lunar cycle (new moon to new moon), while years are adjusted to align with the solar seasons.
• A standard Hebrew month begins with the new moon; months are 29 or 30 days long.
• Because 12 lunar months (~354 days) are about 11 days shorter than a solar year (~365 days), the calendar inserts an extra month in certain years (a leap, or embolismic, year) to stay synchronized with the seasons.

Months of the Hebrew year

The modern rabbinic Hebrew calendar has 12 months in common years and 13 months in leap years. The months, with their usual lengths, are:

• Nisan — 30 days (spring; contains Passover)
• Iyar — 29 days
• Sivan — 30 days (contains Shavuot)
• Tammuz — 29 days
• Av — 30 days (contains Tisha B’Av)
• Elul — 29 days
• Tishrei — 30 days (contains Rosh Hashanah, Yom Kippur, Sukkot)
• Cheshvan (MarCheshvan) — 29 or 30 days (variable)
• Kislev — 29 or 30 days (variable)
• Tevet — 29 days
• Shevat — 30 days
• Adar (in common years) — 29 days
• Adar I and Adar II (in leap years) — Adar I = 30 days, Adar II = 29 days (Adar II is when Purim is observed)

Note: The two-month variability (Cheshvan and Kislev) is used so that the calendar’s fixed rules produce proper weekday placements for major holidays.

Years and leap years

• The basic year length is 12 lunar months (~354 days). To keep holidays in the correct seasons (for example, Passover in spring), the calendar adds an extra month in 7 out of every 19 years — the Metonic cycle. These leap years occur in years 3, 6, 8, 11, 14, 17, and 19 of the cycle.
• A leap year has 13 months; the extra month is called Adar I (Adar I precedes Adar II). Purim is celebrated in Adar II.
• Years are counted from the traditional Anno Mundi (AM) epoch — the calculated creation of the world. As of 2025 the Hebrew year is 5785 (started at sundown on Rosh Hashanah 5785).

Molad and the rules for fixing Rosh Hashanah

The calendar uses a calculated average lunar conjunction called the molad (Hebrew for “birth” of the new moon), specified as days, hours, and chalakim (1 hour = 1080 chalakim). The molad is used in conjunction with fixed postponement (dechiyot) rules to decide the first day of Tishrei (Rosh Hashanah). Key postponement rules include:

• Lo ADU Rosh: Rosh Hashanah cannot fall on Sunday, Wednesday, or Friday.
• Molad Zaken: If the molad of Tishrei occurs at or after noon, Rosh Hashanah is postponed to the next day.
• Other specific rules combine to ensure Yom Kippur and Hoshana Rabbah do not fall on inconvenient weekdays.

These rules make the calendar predictable and allow creation of perpetual calendars without astronomical observation.

Major holidays and when they occur

• Rosh Hashanah (1–2 Tishrei) — Jewish New Year; days of reflection and prayer.
• Yom Kippur (10 Tishrei) — Day of Atonement; fasting and prayer.
• Sukkot (15–21 Tishrei) — Feast of Tabernacles; dwelling in sukkot (booths). Shemini Atzeret and Simchat Torah follow.
• Hanukkah (25 Kislev – ⁄₃ Tevet) — Festival of Lights; eight-day celebration with menorah lighting.
• Purim (14 Adar / 14 Adar II in leap years) — Celebrates the events in the Book of Esther; festive reading, costumes.
• Passover (15–22 Nisan in diaspora; 7 days in Israel) — Commemorates Exodus from Egypt; seder meals mark the start.
• Shavuot (6 Sivan; 7th in diaspora historically) — Festival of Weeks; receiving the Torah.
• Fast of Tisha B’Av (9 Av) — Mourning the destruction of the Temples.
• Minor fasts and commemorative days (e.g., 17 Tammuz, Fast of Gedaliah, Ta’anit Esther) fall on various dates.

Holidays begin at sundown on the evening before the calendar date and run through nightfall on the day listed (timing varies by tradition for start/end).

How date conversion works (conceptually)

• Converting between Gregorian and Hebrew dates requires calculating the Hebrew year’s start (Rosh Hashanah) using molad and postponement rules, then adding months/days.
• Simple patterns: Nisan is in spring (March–April), Tishrei in autumn (September–October). But month-to-Gregorian mapping shifts year-to-year because of leap months and variable month lengths.
• Many online converters use the fixed rules of the rabbinic calendar to compute dates deterministically.

Weekday structure and holiday timing

• The Hebrew week runs Sunday through Saturday, with Shabbat on Saturday (the 7th day).
• Holidays have specific weekday-based restrictions (e.g., when certain holidays fall adjacent to Shabbat, observance details change). The calendar rules prevent certain problematic weekday alignments to keep observance practical.

Cultural and religious significance

• The calendar structures Jewish liturgical year, agricultural festivals, Torah reading cycles, and lifecycle events.
• Names of months derive partly from Babylonian influence after the Jewish exile; older biblical month names and seasonal ties remain.
• The calendar blends astronomical reasoning with rabbinic legal decisions — an intersection of science and religious law.

Practical tips

• For planning: consult a reliable converter or Jewish calendar for exact holiday dates in Gregorian years.
• Know whether you follow Israeli or diaspora customs (some holidays differ by one day).
• When scheduling events, remember Jewish days start at sunset, not midnight.

Common FAQs (brief)

• Q: Why are months 29 or 30 days?
  A: They approximate the lunar synodic month (~29.53 days), so months alternate ⁄₃₀ to average the correct length.
• Q: When is a leap year?
  A: In years 3, 6, 8, 11, 14, 17, 19 of the 19-year Metonic cycle.
• Q: Is the calendar purely astronomical?
  A: No — it’s calculated from traditional rules (molad and postponements), not direct observation.

Further reading and tools

Look up molad calculations, the 19-year Metonic cycle, and perpetual calendar algorithms for technical details. Use established converters and synagogue calendars for precise holiday dates.