Dates and Days

Hebrew calendar

Convert between the Hebrew and Gregorian calendars, either way

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The observances, each beginning at sunset the evening before its daytime date
ObservanceHebrew dateBegins evening ofDaytime dateDays
How this was counted
Hebrew date— Gregorian date— Year length— Leap year— Next observance—

Checked on every build against published Rosh Hashanah and Passover dates and 200 years of the calendar's own rules. How it is checked

Choose a direction and enter a Gregorian or a Hebrew date: the page converts it and lists every observance of that Hebrew year. The observance table gives the evening each one begins as well as its daytime date, since a Hebrew day starts at sunset. The calendar has been fixed arithmetic since the fourth century, so every date here is exact.

Outside Israel, some festivals last a day longer

Passover runs seven days in Israel and eight elsewhere, and Shavuot one day in Israel and two elsewhere. In Israel, Simchat Torah falls on the same day as Shemini Atzeret, so it has no date of its own there. The extra day is called yom tov sheni shel galuyot, and it survives from 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 has two correct lengths. Set Observed to Outside Israel or In Israel, and the Days column of the table gives the length for that choice.

Why the Hebrew calendar is exact

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 arithmetic replaced the sighting of the new moon. The rules set 235 lunar months to every nineteen years. That is the Metonic cycle, and it is close: 235 lunar months come to about two hours more than nineteen solar years. The average lunar month is fixed at 29 days, 12 hours and 793 parts, with 1,080 parts to the hour, and everything else follows by rule.

That is why this page carries no tolerance and the moon phase page does. The Year length readout always shows one of six values: 353, 354 or 355 days in a common year and 383, 384 or 385 in a leap year, labelled deficient, regular or complete. Any other length is an error.

The four postponements, and what they are for

The arithmetic produces a date for the new year, and four rules can then push it later. They are the reason year lengths vary.

The first is usually called lo ADU rosh: the new year is never allowed to start on a Sunday, Wednesday or Friday. 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 other two stop the arithmetic from producing a year of 356 or 382 days, which the calendar does not allow, and they are why a year is sometimes stretched or shortened by a day. Cheshvan and Kislev take up every one of these adjustments, so they are the only two months whose length changes.

A Hebrew day begins at sunset

Ask when Rosh Hashanah falls in 2027 and some sources give 1 October while others give 2 October. Both are right: the first is the evening the observance begins, and the second is its daytime date. The table above prints both for every observance, in the Begins evening of and Daytime date columns. The federal holidays page does the same with statutory and observed dates, because those are also two different facts about one holiday.

Adar, and the month that happens twice

In a leap year the added thirteenth month is a second Adar, so the year has Adar I and Adar II, and Purim, which is kept in Adar, falls in Adar II. Leap years come seven times in every nineteen. When you convert from Hebrew to Gregorian, the Month list offers Adar I and Adar II for a leap year and a single Adar otherwise.

The Islamic calendar is also lunar but has no leap month at all, so it moves backwards through the seasons by eleven days a year.

The Chinese calendar solves the same problem a third way. It is also lunisolar and also adds a leap month seven times in nineteen years, but it picks which month to double from the Sun’s position. That takes astronomy; the Hebrew calendar needs only a fixed rule.

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, and this page prints both, in labelled columns.
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. Rosh Hashanah never falls 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, 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, with Purim in Adar II. 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. That is how the calendar is built, and the converter's month list follows the year from Tishrei round to Elul.