The Jewish Calendar's Months and Years

Zodiac on the mosaic floor of a Byzantine-era Synagogue in Beit Alpha, northern Israel
The central panel of the sixth century Beit Alfa mosaic depicts the lunar Hebrew months as they correspond to the signs of the zodiac.

IAISI/Getty Images

The modern Jewish calendar is the result of centuries of mathematical, astronomical, and religious calculations. The months of the Hebrew calendar, which are based on lunar cycles, are referred to mostly by number in the Bible, but they were also given names almost identical to the names for the Babylonian months.

Key Takeaways: The Hebrew Calendar

  • The Jewish calendar is built on the Babylonian calendar, which the Jews learned to appreciate during the Babylonian Captivity. 
  • The calendar is a lunisolar tool, based on a Metonic combination of the cycles of both the moon and the sun. 
  • It uses a 19-year cycle, that includes seven leap months, rather than the Gregorian's 400-cycle with many more leap days. 
  • The ordinal number of the Hebrew year is the number of Metonic years since the traditional Jewish date of the creation of the world, 3761 BCE.

In the Jewish calendar, each month begins when the moon is just a thin crescent, called Rosh Chodesh, and a new moon in Hebraic tradition. The full moon falls in the middle of each month, and the dark of the moon occurs near the end of the month. When the moon reappears in the sky as a crescent again, a new month begins.

Lunar months are not 30 or 31 days long, as the secular (or "civil") calendar, but rather about 29½ days. The lunar year is 12 months long, or approximately 354 days, eleven days shorter than the solar year of 365 or so. Half days are impossible to factor into a calendar, so the Hebrew calendar is broken down into either 29 or 30 day monthly increments. 

Months on the Jewish Calendar
Hebrew Name Babylonian Name Civic Calendar Length in Days Significant Holidays
Nisan Nisanu March–April 30 Passover
Iyar Ayaru April–May 29 Lag B'Omer
Sivan Simanu May–June 30 Shavuot
Tammuz Diuzu June–July 29  
Menachem Av or Av Abu July–August 30 Tisha B'Av
Elul Ululu  August–September 29  
Tishri or Tishrei Tashritu September–October 30 Rosh Hashanah and Yom Kippur
Chesvan or Marchesvan Arakhasamna October–November 29 or 30  
Kislev Kislim November–December 29 or 30 Chanukah begins
Tevet Tebetru December–January 29 Chanukah ends
Shevat Shabatu January–February 30 Tu B'Shvat
Adar Adaru  February–March 30 Purim
Adar Beit   (Leap Month) 29  
Information about the months in the Hebrew calendar.

The rabbis who first began working out the Jewish calendar in the fourth century CE recognized that limiting all months to either 29 or 30 days wasn't going to work. Two months were then given a bit more flexibility, Cheshvan and Kislev.

Babylonian Names

The main purpose of any calendar is to know when to plant crops, the most vital piece of knowledge in the universe for a farmer. Too early, crops are nipped by frost; too late, crops don't ripen. Either way, the community suffers great losses. 

The basics of the Jewish calendar were acquired during the 6th century BCE "Babylonian captivity" of the Hebrews. Dates and details of that time are debated, but in essence, the Neo-Babylonian ruler Nebuchadnezzar II attacked Jerusalem, conquered Judah, dismantled the Temple of Solomon, and deported perhaps one-quarter of the Jews to Babylon. 

The Jerusalemites in Babylon included the king Jeconiah, his court and perhaps as many as 20,000 others, including the prophet Ezekiel. There they stayed for about 50 years until Babylon was conquered by the Persian Cyrus the Great in 539 BCE. Cyrus set the Hebrews free to go home but made Judah a province of the Persian empire. 

Setting the Jewish Year 

The Babylonian calendar was a lunisolar tool which had about 354 days divided into 12 lunar months, with seven day weeks. Each month began when a crescent moon was first sighted in the sky—if the sky was cloudy, you had to wait until the next night. There were astronomical, mathematical, and religious reasons why the Babylonian calendar wouldn't do. 

Today, more than 2,600 years later, we know: 

  1. The earth's solar year lasts 365.2422 days.
  2. Our lunar cycle lasts 29.53059 days.
  3. To get the right dates for planting you need both. 

That level of precision looks bizarrely detailed for somebody without a calculator, but it was readily apparent to farmers when it fell short. On top of the imprecision, there are religious complications. 

For example, Rosh Hashanah must begin on a new (crescent) moon, on the first day of the month of Tishri; Passover begins on the 15th of Nisan. Whatever you call the month, Passover must fall in the spring and Rosh Hashanah must begin in the fall, a half year later. Passover also has to have a full moon on the night of the first seder, and there must be a full moon on the first night of Succoth on the 15th of Tishri. There are other requirements as well.

Transitioning to a Fixed Calendar

After returning to Jerusalem, the Hebrews continued to use the Babylonian calendar for about a century, then they established a Calendar Council (Sod Hadibbur in Hebrew), consisting of the president plus two to six members of the Sanhedrin who were skilled in astronomy and mathematics. For the next 800 years, until the mid-4th century CE, the Calendar Council set the religious and secular calendar up for the Jews of Jerusalem and the growing diaspora. Every month, they were tasked with setting the first day of each month by direct observation of the moon's phases, and determining whether the extra "leap month" was required to maintain the balance between solar and lunar year. 

Over the 800 years, different rules and adjustments were made. In the 3rd century CE, new rules said that the first day of Rosh Hashanah cannot fall on a Sunday, Wednesday or Friday so that Yom Kippur wouldn't fall on or near the Sabbath. By the early part of the 4th century, Rabbi Hillel II (died 365 CE) put a fixed calendar in place, so that people would know in advance when the festivals would occur, and when they could more or less safely plant crops.  

Jewish Leap Years: A 19 Year Cycle

To correct for the quarter day extra in a solar year, the Gregorian calendar has a 400-year cycle that adds an additional "leap day"—February 29—to every year that is divisible by four. Even in a nineteen-year cycle, you still need to correct for the imprecision and to realign the calendar so that Passover falls in spring, which the Hebrew scholars do by adding an extra month to the calendar.

In the fifth century BCE, the Greek astronomer Meton (d. 460 BCE) pointed out that the number of days in 19 solar years is almost exactly the same number of days in 235 lunar cycles, a total of 6939.6 days (235 x 29.53059) / (19 x 365.2422) = 6939.689 / 6939.602 = 1.000013). His resulting Metonic cycle is what the Hebrews ended up using—as did the Babylonians, who knew of the Metonic cycle before Meton was born. 

In other words, over a 19-year period, each Hebrew year varies in length from 353 to 385 days. A 13th month is added at the end of the year seven times in each 19-year cycle: the third, sixth, eighth, 11th, 14th, 17th and 19th years of each cycle has an extra month, which is called Adar Beit. It follows "Adar I" and lasts 29 days. 

Dates of the Current 305th Cycle
Ordinal No. Year Civil Date of 1st Tishri No of days
1 5777 Monday, 3 October 2016 354
2 5778 Thursday, 21 September 2017 355
3 5779 Monday, 10 September 2018 385
4 5780 Monday, 30 September 2019 353
5 5781 Saturday, 19 September 2020 354
6 5782 Tuesday, 7 September 2021 385
7 5783 Monday, 26 September 2022 355
8 5784 Saturday, 16 September 2023 383
9 5785 Thursday, 3 October 2024 354
10 5786 Tuesday, 23 September 2025 355
11 5787 Saturday, 12 September 2026 383
12 5788 Saturday, 2 October 2027 354
13 5789 Thursday, 21 September 2028 355
14 5790 Monday, 10 September 2029 385
15 5791 Saturday, 28 September 2030 354
16 5792 Thursday, 18 September 2031 353
17 5793 Monday, 6 September 2032 385
18 5794 Saturday, 24 September 2033 354
19 5795 Thursday, 14 September 2034 385
The varying lengths of the Jewish calendar years.

Dates in the Jewish Calendar

The Jewish year is numbered differently from the Gregorian, of course. For one thing, the Gregorian calendar year numbers start with the supposed birth year of the Christian leader Jesus Christ, and the Jewish church is much older than that. 

Currently, the Jewish calendar is in the 305th 19-year cycle, which runs from 2016 through 2035. According to Jewish tradition, the world was created in the autumn of 3761 BCE (and not, as according to Christian tradition, in the autumn of 4004 BCE); the 305th cycle since creation began in September of 2017, or 5777 years after the creation. The precise date of the creation was first established in the 12th century, by the Jewish philosopher Maimonides (1135–1204): the Jewish year which began in October of 2016 and ended in September of 2017, was the year 5777.

Is Calendar Making a Rocket Science?

The invention of a reliable, fixed calendar is a complex and difficult task that took millennia to get right. The efforts of the Bronze Age Greeks, Egyptians, Mesopotamians, and Judaen cultures to make sense of the world's seasonality is arguably the linchpin for all the science (and religion, too) that followed.

Sources