This guide explains how the hebrew calendar adds a whole leap month in plain language, with the exact figures and how to work them out yourself.
Last updated: August 1, 2026
The Hebrew calendar attempts something structurally hard: months that follow the moon and a year that stays loyal to the seasons. Twelve lunar months total about 354 days, eleven short of the solar year — left alone, Passover would drift backward through winter into autumn, as Ramadan does in the purely lunar Islamic system. The Torah, however, fixes Passover in spring, so the drift must be repaid.
The repayment is the leap month. In seven years of every nineteen — years 3, 6, 8, 11, 14, 17, and 19 of the cycle — an entire thirteenth month is inserted: a second Adar, called Adar I, slotted before the regular Adar (which becomes Adar II, keeping Purim and other Adar observances in the month adjacent to Nisan and Passover). Nineteen solar years happen to equal almost exactly 235 lunar months — the Metonic cycle, known to Babylonian and Greek astronomers — so the pattern self-corrects with remarkable precision, wandering less than a day against the moon over centuries.
The calendar’s cleverness runs deeper still. Two ordinary months, Heshvan and Kislev, flex between 29 and 30 days, letting a year run 353, 354, or 355 days (383–385 in leap years). These adjustments serve the famous postponement rules: Rosh Hashanah, the new year, may not fall on Sunday, Wednesday, or Friday — placements that would push Yom Kippur against Shabbat or create other observance conflicts — so the calendar quietly shifts the year’s start a day when needed.
Remarkably, all of this runs on fixed calculation rather than observation. Ancient practice declared months by witnesses sighting the new crescent from Jerusalem; the calculated system attributed to Hillel II in the fourth century CE replaced testimony with arithmetic, allowing scattered communities worldwide to keep identical dates for seventeen centuries without a central authority announcing a thing.
The result: Jewish holidays wobble within a window of about a month on the civil calendar — Hanukkah anywhere from late November to late December, Passover across March and April — but never abandon their seasons.
Today’s exact Hebrew date appears live in the world calendars panel at itsdately.com, alongside a holiday list that computes Rosh Hashanah, Passover, and Hanukkah from the calendar’s own rules — Metonic mathematics still on duty.
The system’s accuracy has one slow leak worth noting: the calculated year averages about 365.2468 days, a few minutes longer than the true solar year, so the calendar drifts roughly a day later every couple of centuries — Passover creeps very slowly deeper into spring. Rabbinic authorities have long been aware; any correction awaits a future consensus, and at the current rate the question remains comfortably academic for many generations.
Daily life adds a final elegant rule: the Hebrew day begins at sundown, not midnight, following Genesis’s “and there was evening and there was morning.” Every holiday therefore opens the evening before its calendar date — candles lit as the sun sets — which is why printed Jewish calendars carry the reminder that observances begin the previous night, and why this site’s holiday list notes the same.
Frequently asked questions
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Related on itsdately: Why New Year Begins on January 1 · How Leap Years Work in Other Calendars · The Hindu Panchang: Five Measurements in Every Single Day
Further reading: Hebcal — the Jewish calendar explained For readers asking “How the Hebrew Calendar Adds a Whole Leap Month”, the answer comes down to the specific factors covered above.
Related read: From Milliseconds to Attoseconds: The Small Units of Time.