In Israel every date is written twice: once on the Hebrew calendar and once on the Gregorian one. This page explains the difference between them, why the festivals "move" against the civil calendar from year to year, and how to read a combined calendar in which every cell carries both dates.
Every day in Israel carries two dates at once. The invoice is due on 3 February, the wedding is set for the 11th of Shevat, and both name the same square on the wall. Most of the time the two run quietly in parallel — until you have to cross-reference them, and work out which civil date a yahrzeit falls on this year, or which day to book off work.
The gallery above lays out the coming months one after another, each cell carrying both dates; click any month to open it full size. Below it, this page explains what sits behind that cross-reference: why the two calendars move at different speeds, and how to read them side by side without getting lost.
The difference between the two calendars starts with a single question: what is being counted.
The Gregorian calendar counts the Sun. Its year is the time the Earth takes to go once around, roughly 365 and a quarter days, and the division of that year into months is purely administrative — thirty-one days, thirty, and February with whatever was left over. It could have been carved up differently. Nothing in a civil month corresponds to anything in the sky.
The Hebrew calendar counts the Moon. A month is one complete lunar cycle, about twenty-nine and a half days, and Rosh Chodesh falls close to the new moon. That is why a Hebrew month runs either twenty-nine days or thirty and never lands in between: a day cannot be cut in half. And unlike a civil month, a Hebrew one is visible. Step outside at night, look up, and you can tell roughly where in the month you are standing.
Twelve lunar months add up to about 354 days — some eleven days short of a solar year. Almost everything else on this page follows from that gap.
Those eleven days are why a civil birthday returns to precisely the same date each year while a Hebrew date wanders. Each year the Hebrew date arrives about eleven days earlier against the civil calendar; after a leap year, which has absorbed a whole extra month, it drops back by about nineteen.
A purely lunar calendar simply lives with the gap. The Islamic calendar makes no correction, so Ramadan travels through all four seasons over a cycle of some thirty-three years, and a single lifetime can see it fall in high summer and in deep winter. The Hebrew calendar has no such freedom, because one explicit obligation is placed on it: "Observe the month of Abib and offer a passover sacrifice to the ETERNAL your God…" 1. Passover has to fall in spring, so the calendar is obliged to keep step with the Sun.
The practical consequence is a tidy one: any Hebrew festival moves within a window of about a month against the civil calendar, and no further. Passover falls between late March and late April, Rosh Hashanah between early September and early October, Chanukah between late November and late December. The date itself is going nowhere; the two rulers just refuse to line up.
The fix is almost disappointingly plain: once the shortfall has built up to roughly a month, a month is added.
Seven times in every nineteen years the Hebrew year takes on an extra month and runs to thirteen. The number nineteen is not arbitrary, and its accuracy is the striking part — nineteen solar years come to about 6,939.6 days, and two hundred and thirty-five lunar months come to about 6,939.7. After one full cycle the two calendars have returned to very nearly the same point, about two hours apart.
That is also behind something people tend to notice by accident: once every nineteen years, a Hebrew birthday and a civil birthday land back on the same day.
The extra month is not tacked onto the end of the year — it goes in ahead of Adar. A leap year has Adar I and Adar II, and Adar II is the one that carries the festivals: Purim is kept in it, as is the bar mitzvah of anyone born in the Adar of an ordinary year.
A question that surfaces every time somebody converts a date: if the year opens at Rosh Hashanah, which falls in Tishrei, why does the Torah call Nisan "the first month"?
Both reckonings are correct — they are simply measuring different things, and the Mishnah opens with exactly this point: "They are four days in the year that serve as the New Year… On the first of Nisan is the New Year for kings… On the first of Tishrei is the New Year for counting years…" 2.
Nisan starts the count of months, so it comes first when you are numbering months. Tishrei starts the count of years, so that is where the number changes. For converting dates this is the most useful detail on the page: the Hebrew year number turns over in autumn — not in January, and not in spring. A festival falling in September or October already belongs to the incoming Hebrew year, and the spring festival that follows it, in April of the next civil year, still carries that very same Hebrew year.
In a combined calendar every cell carries two dates: the civil one in digits, and the Hebrew one in letters — alef, bet, gimel and onward to lamed. The Hebrew month name appears on the day it changes over, so you can see on the page exactly where one Hebrew month ends and the next begins. And because the months do not overlap, a Hebrew month almost always straddles two civil ones.
The one detail worth committing to memory is the changeover point. The civil day begins at midnight; the Hebrew day begins at sunset. An evening therefore already belongs to the following day’s Hebrew date: Friday night carries the Hebrew date of Shabbat even though the civil calendar still says Friday. The same rule governs when a festival starts, when the Chanukah lights are kindled and when a fast begins — which is why any holiday table lists the eve of a festival separately from the day itself.
The same days can be sorted two ways, depending on what you need. On the monthly civil calendar the month opens on the 1st and closes on the last, with the Hebrew date alongside each day. On the Hebrew month calendar it is the other way round — the month opens at Rosh Chodesh and closes at its end, and the civil date is the one riding along.
How many months are in a Hebrew year?
Twelve in an ordinary year and thirteen in a leap year. The additional month is Adar I, and it is inserted ahead of the regular Adar.
Why are some Hebrew months only twenty-nine days?
Because the lunar cycle runs about twenty-nine and a half days, and a day cannot be split. Full months of thirty and deficient months of twenty-nine alternate, and Cheshvan and Kislev can shift between the two to balance the year.
Does the Hebrew date change at midnight?
No — it changes at sunset. Someone born in the evening, after sundown, was already born on the following day’s Hebrew date.
How often is there a leap year?
Seven times in every nineteen years. The spacing is uneven, so two leap years can be either two or three years apart.
A screen shows one date at a time. A printed sheet shows a whole month, with both dates sitting beside each other on every day of it — which is the difference between converting a single date and watching the two calendars travel together across a month.
Observe the month of Aviv and offer a Passover sacrifice to the LORD your God…source
They are four days in the year that serve as the New Year… On the first of Nisan is the New Year for kings… On the first of Tishrei is the New Year for counting years…source