Dates and Days

Hebrew calendar

Convert between the Hebrew and Gregorian calendars, either way

The Hebrew calendar is the one calendar on this site that needs no astronomy at all: it has been fixed arithmetic since the fourth century, so every date here is exact rather than accurate to within something. Both directions convert, and every observance of the year is listed with the evening it begins — because a Hebrew day starts at sunset, and that is where most calculators quietly pick one answer.

Hebrew date— Gregorian date— Year length— Leap year— Next observance—
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The observances — each begins at sunset the evening before its daytime date
ObservanceHebrew dateBegins evening ofDaytime dateDays

Outside Israel, some festivals last a day longer

Passover runs seven days in Israel and eight elsewhere; Shavuot is one day and two; Simchat Torah is not a separate date in Israel at all, but is kept together with Shemini Atzeret. The extra day is called yom tov sheni shel galuyot and it is a survival of the era before a fixed calendar: the new month was declared in Jerusalem and announced outward by signal fires and messengers, so communities far enough away could not be certain which day had been declared and kept both. The calendar stopped depending on that announcement seventeen centuries ago and the practice remained.

So the same festival genuinely has two correct lengths, and a calculator that shows one without saying which is answering a question you did not ask. The control above chooses; the table says how many days either way.

Why this one is exact and the others are not

Most calendars that track the Moon have to observe it. The Hebrew calendar stopped doing that in the fourth century, when the rules were written down and the sighting of the new moon was replaced by arithmetic. Nineteen years are declared to hold exactly 235 lunar months — the Metonic cycle, and it is close enough that the error is about two hours a century — and the mean interval between new moons is fixed at 29 days, 12 hours and 793 parts, where a part is one 1080th of an hour. Everything else follows by rule.

That is why this page carries no tolerance and the moon phase page does. There is nothing here to approximate, so there is nothing to be within. A year is 353, 354, 355, 383, 384 or 385 days long and never anything else; if a calculator gives you a different number it is not slightly off, it is wrong.

The four postponements, and what they are for

The arithmetic produces a date for the new year and then four rules may push it later. They are worth knowing because they are the whole reason year lengths vary.

The first is usually called lo ADU rosh: Rosh Hashanah may not fall on a Sunday, Wednesday or Friday. The reason is knock-on — those weekdays would place Yom Kippur immediately beside a Sabbath, giving two consecutive days on which no work may be done, or put Hoshana Rabbah on a Sabbath. The second, molad zaken, moves the year on when the calculated new moon falls at or after midday. The remaining two exist to prevent year lengths the system does not allow, and they are why a year is sometimes stretched or shortened by a day. All of that slack is absorbed in two months, Cheshvan and Kislev, which is why those two are the only ones whose length changes.

A day begins at sunset, and that is not a detail

Ask when Rosh Hashanah falls in 2027 and you will be told two different dates depending on who you ask, and both are right. The observance begins at sunset on 1 October 2027 and its daytime date is 2 October. Newspapers usually print the evening; a Hebrew calendar prints the day. This page prints both, in labelled columns, for every observance — which is the same choice the federal holidays page makes about statutory and observed dates, for the same reason: they are two different facts and collapsing them is the error that actually costs someone something.

Adar, and the month that happens twice

The Islamic calendar is the instructive contrast: also lunar, but with no leap month at all, so it walks backwards through the seasons by eleven days a year instead of staying locked to them. Two lunar calendars, opposite answers to the same problem.

Seven years in every nineteen are leap years, and a Hebrew leap year adds a whole month rather than a day. The added month is a second Adar, so a leap year runs Adar I then Adar II. Purim falls in Adar — and in a leap year it falls in Adar II, not the first one, which is the single most common thing to get wrong. The converter names the months correctly for whichever kind of year you land in, and the month list changes accordingly.

One more thing that looks like a bug and is not: the months are numbered from Nisan, which the Torah calls the first month, while the year number changes at Tishrei, which is the seventh. So 1 Tishrei 5788 comes before 15 Nisan 5788 by about six months. That is the calendar, not a mistake in it.

Questions

Why does Rosh Hashanah have two different dates?
Because a Hebrew day begins at sunset, so every observance starts the evening before its daytime date. Rosh Hashanah 5788 begins at sunset on 1 October 2027 and its daytime date is 2 October 2027. Newspapers usually print the evening; a Hebrew calendar prints the day. Both are correct, they are answers to different questions, and this page prints both in labelled columns rather than choosing one for you.
Is this calculation exact, or approximate?
Exact. The Hebrew calendar stopped depending on observing the new moon in the fourth century and has been fixed arithmetic ever since: nineteen years are declared to hold exactly 235 lunar months, and the interval between new moons is fixed at 29 days, 12 hours and 793 parts, where a part is one 1080th of an hour. There is no ephemeris involved and therefore no tolerance. A Hebrew year is 353, 354, 355, 383, 384 or 385 days long and never anything else.
Why do Hebrew year lengths vary by up to two days?
Because four rules can postpone the new year, and the slack has to go somewhere. Rosh Hashanah may not fall on a Sunday, Wednesday or Friday, since those placements would put Yom Kippur beside a Sabbath or Hoshana Rabbah on one. A new moon falling at or after midday also moves the year on. Two further rules exist to prevent year lengths the system does not allow. All the resulting adjustment is absorbed by two months, Cheshvan and Kislev, which is why only those two change length.
Why is Purim in Adar II in some years?
Because a Hebrew leap year adds a whole month rather than a day, and the added month is a second Adar. Seven years in every nineteen are leap years, and in those the year runs Adar I then Adar II. Purim falls in Adar II, not the first one. This is the most commonly mistaken point in the whole calendar, and the converter names the months correctly for whichever kind of year you land in.
Why does month 1 come after month 7 in the same year?
Because the months are counted from Nisan, which the Torah calls the first month, while the year number changes at Tishrei, which is the seventh. So 1 Tishrei 5788 falls about six months before 15 Nisan 5788. It looks like an error in a converter and it is not; it is how the calendar is built, and any tool that hides it will be wrong somewhere else instead.