How the calendar works

Rosh Chodesh & the Jewish month

Why the calendar follows the moon, how it is kept from drifting out of the seasons, what happens on the first of the month, and why your birthday moves around every year.

• The moon, and the problem it creates
• The fix: a thirteenth month
• The twelve months
• How the month used to be declared
• What actually happens on Rosh Chodesh
• Your own dates

The moon, and the problem it creates

A Jewish month is a real lunar cycle: new moon to new moon. That interval — the molad — is 29 days, 12 hours and 793 parts of an hour, a figure the rabbis of the Talmud used and which is accurate to within about half a second of the modern measurement.

Since a month cannot have half a day in it, months alternate between 29 and 30 days. Twelve of them make about 354 days. A solar year is about 365¼. So every year the lunar calendar runs eleven days short.

Left uncorrected, that drift is fast: Passover would move backwards into winter within a decade, and round the whole year in thirty-three. The Islamic calendar allows exactly this and Ramadan travels through every season. The Jewish calendar cannot, because the Torah requires Passover to be kept in chodesh ha’aviv — the month of spring.

The fix: a thirteenth month

Seven times in every nineteen years, a whole extra month is inserted. Nineteen solar years and 235 lunar months come out almost exactly equal — a correspondence known to Babylonian astronomers and named in Greek sources after Meton of Athens. Seven leap years per nineteen brings the two calendars back into step within about two hours.

The leap years are years 3, 6, 8, 11, 14, 17 and 19 of the cycle, and the added month is a second Adar. It is the second Adar that is the real one for practical purposes: Purim is kept in it, a yahrzeit for someone who died in an ordinary Adar is generally observed in it, and a boy born in an ordinary Adar becomes bar Mitzvah in it. Adar I gets a small consolation prize called Purim Katan.

The twelve months

Counted from Nisan, which the Torah calls the first month — even though the year number changes at Tishrei, six months later. The Mishnah lists four different new years for four different purposes, and finds nothing odd about it.

Cheshvan and Kislev are the two that change length, and they are the calendar’s adjustment screws. By making one or both a day longer or shorter, the year is set to 353, 354 or 355 days — or, in a leap year, 383, 384 or 385. That is done to satisfy a set of rules called the dechiyot: Rosh Hashanah may not fall on a Sunday, Wednesday or Friday, which among other things keeps Yom Kippur from landing next to Shabbat and Hoshana Rabbah from landing on it.

The month names, incidentally, are Babylonian. They came back with the exiles in the sixth century BCE; the Torah itself mostly numbers the months rather than naming them.

How the month used to be declared

None of this used to be arithmetic. Two witnesses who had seen the new crescent came to the court in Jerusalem and were cross-examined. If the court accepted them, it declared: mekudash, sanctified. Beacons were lit on hilltops to carry the news north, until opponents began lighting false ones and the system was replaced by messengers on foot.

Which is where the second festival day comes from. A community far enough away that the messenger had not arrived could not know whether the month had begun on the thirtieth day or the thirty-first — and therefore could not know which of two days was Passover. So they kept both. Around the fourth century Hillel II published a fixed calculated calendar, and the uncertainty ended; the second day was kept anyway, by rabbinic decision, and is kept still everywhere outside Israel.

That is why Rosh Hashanah is two days everywhere including Israel — it is on the first of the month itself, so even Jerusalem could not always know — and why Simchat Torah is a separate day outside Israel and not inside it.

What actually happens on Rosh Chodesh

Rosh Chodesh is a minor festival. Work is permitted — you go to the office — but the day is marked:

• Ya’aleh v’yavo is added to the Amidah and to grace after meals.
• Half Hallel is said in the morning.
• A Musaf service is added, and there is a short Torah reading.
• No tachanun, and no fasting or eulogies.
• A slightly better meal. A modest obligation, easily kept.
• A semi-holiday for women — a long-standing custom of refraining from certain work, on the strength of the midrash that the women refused to contribute to the Golden Calf.

When the outgoing month had thirty days, Rosh Chodesh is two days: the thirtieth of the old month and the first of the new. When it had twenty-nine, it is one.

Shabbat Mevorchim

On the Shabbat before each Rosh Chodesh (except before Tishrei), the coming month is announced and blessed in Shul, with the exact time of the molad read out to the minute. Chabad custom adds something specific to that Shabbat: the whole book of Psalms is recited in the morning before prayers, and there is usually a farbrengen afterwards.

Kiddush Levana

Once a month, outdoors, under an open sky, the new moon is blessed — a short service said standing, looking up, usually on a Saturday night after Shabbat has ended. It can be said from a few days after the molad (Chabad custom waits seven) until roughly the middle of the month. It is one of the few pieces of Jewish practice that requires you to go outside and look at something.

Your own dates

A Hebrew birthday, a bar or bat Mitzvah date and a Yahrzeit are all reckoned on this calendar, which is why they land on a different civil date every year and why a Yahrzeit can arrive a fortnight earlier than you expected. Working it out by hand is unnecessary:

• The yahrzeit page converts a date and will send a reminder each year.
• The times page shows today’s Hebrew date and every holiday coming up in the next six months.
• The community calendar shows what is actually happening here.

A last observation, because it is the point of the whole design. A purely solar calendar keeps the seasons and loses the moon. A purely lunar one keeps the moon and loses the seasons. This one refuses to give up either, and pays for that with a thirteenth month seven times in nineteen years. It is more work than the alternatives, and it is the reason Passover is in the spring and Chanukah is in the dark.

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