The Jewish Calendar: IAQ & A – Infrequently Asked Questions, and Answers

By Jonathan Berkowitz


Q. What is the first month of the year?

Nisan is the first month on the Jewish calendar. Before the Jews left Egypt, on the first day of the month of Nisan, the Torah says, “The Lord spoke to Moses and to Aaron in the land of Egypt, saying, ‘This hodesh shall be to you the head of months.’” (Exodus 12:1–2) That introduces a peculiarity to the Jewish calendar. The year (administrative counting) begins on Rosh Hashanah, the first day of the month of Tishrei (the anniversary of the creation of Adam and Eve), but Tishrei is not the first month. In the Torah, Rosh Hashanah is referred to as “the first day of the seventh month.”


Q. Why is Rosh Hashanah, the so-called Jewish New Year, the first day of the seventh month, not the first day of the first month?

The Jewish calendar has several distinct new years, each used for a different purpose. This is analogous in modern times to different start dates for fiscal, tax, and academic years, etc. Hence, 1 Nisan is the new year for kings and the ecclesiastical new year, from which date months and festivals are counted. That’s why the Torah says that Passover, which begins on 15 Nisan, comes in the first month while Rosh Hashanah, which begins on the 1 Tishrei, comes in the seventh month. That means 1 Tishrei is the start of the civil new year, and the date when the year number increases by one. It also marks the end of one agricultural year, so is the new year for agriculture-related commandments such as Shmita (release) and Yovel (Jubilee). In addition, 1 Elul is the new year for the cattle tithe, and 1 Shevat is the new year for trees.


Q. Why does Rosh Hodesh have semi-festive status?

Since Jewish festivals are scheduled according to the days of the month, it has always been important to know exactly when the month begins. Based on the Torah verse mentioned above, the first day of the month, and the thirtieth day of an abundant month (see next item for details), are called Rosh Hodesh, the “Head of the Month.” The sages interpreted the wording, “shall be to you,” to mean that pinpointing and consecrating the Hodesh, the crescent new moon, was the responsibility of the Sanhedrin, the rabbinical supreme court. It remained their job until the fixing of the calendar in the fourth century CE and the dissolution of the Sanhedrin.


Q. Why is Rosh Hodesh sometimes one day and sometimes two?

The Sanhedrin determined the months aided by testimony of witnesses claiming to see the thin crescent new moon, which was the signal for a new Jewish month. When the Sanhedrin assembled on the thirtieth of each month, the day would be designated Rosh Hodesh of the upcoming month, which meant the previous month had 29 days. But if it took a day longer, and therefore was a 30-day month, the next day would also be Rosh Hodesh.


Now, with our fixed calendar, for a 29-day month, the Rosh Hodesh of the following month will be only one day. For a 30-day month, the thirtieth day and the first day of the next month are observed together as Rosh Hodesh. Thus, the months of Heshvan, Adar (and Adar II), Iyar, Tammuz, and Elul always have two days of Rosh Hodesh (the first day of

the month plus the last day of the previous month). The months of Tishrei, Shevat, Nisan, Sivan, and Av always have one day of Rosh Hodesh. What about Kislev and Tevet? Because their length varies (29 or 30 days), in some years they both have a one-day Rosh Hodesh, some years both have two days, and some years Kislev is a one-day and Tevet is a two-day Rosh Hodesh. And we skip one month of celebration; Rosh Hodesh per se is not celebrated in Tishrei due to Rosh Hashanah. The head of the year takes precedence over the head of the month!


Q. Why are some holidays in the Diaspora two days but only one day in Israel.

Once the Sanhedrin had determined that a new month had begun, the information was broadcast from Jerusalem to distant Jewish outposts via huge bonfires which were lit on designated mountaintops. Lookouts stationed on other mountaintops would see the fires, and would then light their own fires, creating a chain of communication that led all the way to Babylon and beyond. If there was a Yom Tov that month, communities across Israel and in the Diaspora would then know when to celebrate it. But a problem arose. The Samaritans, a sect that denied rabbinic authority and were constantly at odds with the Jews, started lighting fires on the wrong days to manipulate the calendar. To prevent confusion, the fire-on-mountaintop method of communication was discontinued; instead, messengers were dispatched to Babylon and other far-flung Jewish settlements. Since news traveled much more slowly that way, distant communities would not know when Rosh Hodesh had been declared in time to celebrate the festival on the proper day. It was therefore decreed that, outside of the Land of Israel, people would celebrate every Yom Tov for two days: the day of the month the holiday would be if the previous month had been a 29-day month, and the day of the month it would be if the previous month had been a 30-day month.


Q. How long is a Jewish day? And when does a Jewish day begin?

When God created time, night was created before day. “And it was evening and it was morning, one day.” (Genesis 1:5) That’s why a Jewish calendar date starts the night before. (A case in point in Leviticus 23:32, Yom Kippur is defined as lasting “from evening to evening”.) A day in the secular calendar starts and ends at midnight, but a Jewish day goes from nightfall to nightfall. The challenge is that the exact moment at which night begins is not clear. The twilight period, from sunset until three stars are visible in the sky is uncertain and inconsistent—for example, what happens if the sky is cloudy? Shabbat and all the holidays begin at sunset, the earliest possible definition of nightfall, and end when three stars appear in the sky the next evening, the latest definition of nightfall, so days are defined locally.


While a secular day is always 24 hours (except for adjustments for daylight savings time), the length of a Jewish day differs from one day to the next! The length of days (and hours—see the next item) vary by the season, and are controlled by the times of sunset, nightfall, dawn and sunrise. It’s complicated! There are also various opinions, in addition to the international dateline convention, about where in the world the day changes.


Q. How long is a Jewish hour?

The hour has a special meaning in Jewish law. Halachically, an hour is calculated by dividing the total time of daylight of a particular day, from sunrise to sunset, into twelve equal parts. Thus, it is a proportional hour (sha’ah zemanit), not a sixty-minute hour. It varies by the season and by the day.


To illustrate, consider a day when sunrise is at 5:00 am and sunset is at 7:30 pm and therefore is 26.5 hours long. Then one proportional hour will be 14.5 hours divided by 12 or 72.5 minutes long. The third hour of the day ends at 8:37:30 am (5:00 am plus 3×72.5 minutes). These details matter because many observances in Jewish law, such as daily prayer services, are performed at specific times during the day. The calculation of these halachic times depends on the length of the daylight hours in that locale. Not surprisingly, there are other systems too, but they are not used for everyday purposes. We rely on the local civil clock for reference since Jewish measurements of time do not use clocks.


Q. On which days of the week do Jewish holidays begin?

The structure of the Jewish calendar restricts which days of the week holidays can begin.


Passover: The first Seder can be on the evenings of Friday, Saturday, Monday, or Wednesday. This means the first day of Passover can be Shabbat, Sunday, Tuesday, or Thursday. The first Seder never falls on the evenings of Sunday, Tuesday, or Thursday, so the first day of Passover is never Monday, Wednesday, or Friday. There is a mnemonic to assist: Lo bedu Pesach (לא בד”ו פסח), Passover never starts on bedu, an acronym for bet (Monday), dalet (Wednesday) or vav (Friday).


Shavuot: It comes exactly seven weeks after the second day of Passover. For example, if Passover begins on Sunday, Shavuot will begin on Monday. Therefore, Shavuot can begin at sundown on Saturday, Sunday, Tuesday, or Thursday, meaning that the first day of the holiday can be Sunday, Monday, Wednesday, or Friday. Conversely, Shavuot never begins on Monday, Wednesday, or Friday nights, so the first morning of Shavuot is never Tuesday, Thursday, or Shabbat. There is a mnemonic for this as well: Lo gehaz Shavuot (לא גה”ז שבועות), Shavuot does not start on gimel (Tuesday), heh (Thursday) or zayin (Shabbat).


Rosh Hashanah and Yom Kippur: The first day of Rosh Hashanah is always the same day of the week as the third day of Passover. So, Rosh Hashanah can begin at sundown on Sunday, Monday, Wednesday, or Friday, which means that the first day of Rosh Hashanah can be Monday, Tuesday, Thursday, or Shabbat. Conversely, Rosh Hashanah never begins on Shabbat, Tuesday, or Thursday nights, so that the first morning of Rosh Hashanah is never Sunday, Wednesday, or Friday. There is yet another mnemonic: Lo adu Rosh (לא אד”ו ראש), Rosh Hashanah does not start on aleph (Sunday), dalet (Wednesday) or vav (Friday).


Once you know the days Rosh Hashanah is celebrated, you can quickly calculate that Yom Kippur is exactly one week after the day after the second day of Rosh Hashanah. Thus, if Rosh Hashanah is on Monday and Tuesday (for example), Yom Kippur will be on Wednesday (beginning at sundown on Tuesday). Note also that Sukkot and Shemini Atzeret both begin on the same day of the week as the first day of Rosh Hashanah, so calculating when they fall is simple.


Calculating the Entire Year

A famous cipher (substitution code) is known as Atbash. It works by pairing up the first letter of an alphabet with the last letter, the second letter with the second-last, and so on. For the English alphabet that would code A as Z, B as Y, C as X, and so on, to Z as A. In Hebrew that would mean the first letter (aleph) is paired with tav (the final letter), and the second letter (bet) is associated with the penultimate letter (shin), etc. And that’s where the name comes from: Aleph, Tav, Bet, SHin.


Starting with the day of the week when Passover begins in any given year, one can then calculate when many (but not all) of the year’s holidays will be celebrated. Here’s how.

Aleph (the first day of Pesach) is the same day of the week as tav, (17 Tammuz and) Tisha B’Av.

Bet (the second day of Passover) is the same day of the week as shin, Shavuot.

Gimmel (the third day) is the same as resh, Rosh Hashanah (and Sukkot and Shemini Atzeret).

Dalet (day four) give the day of the week for kuf, which stands for kriah (“reading”), an allusion to Simchat Torah, when the Torah reading cycle begins and ends in the Diaspora.

Hay (day five) corresponds to tzaddi, tzom, the fast day of Yom Kippur.

Vav (day six) corresponds to pei for the Purim celebrated five weeks earlier (but not the next Purim!).

Zayin (day seven) pairs up with ayin, standing for etzim (“wood”), the upcoming minor holiday of 15 Av, which (among other things) marked the end of the season of cutting wood for the Temple altar.


You’re on your own for the dates of the upcoming holidays of Purim and Hanukkah, since the intervening months of Heshvan and Kislev have a variable number of days, and since Purim sometimes comes after an extra month of Adar has been inserted.


Questions for another time:

  • Where did the names of Jewish months come from?
  • What plans are in place for adjustments due to the calendar drift?
  • Who oversees the Jewish calendar!
  • Historically, were there disputes about how the calendar should be set up? Did all communities agree with the rabbinic principles?