Jewish Calendar

לוּחַ עִבְרִי
LOO-akh eev-REE<br/>Hebrew Calendar

Level: Basic

  • Based on moon cycles instead of sun cycles
  • "Leap months" are added to sync up with sun cycles
  • Used to be calculated by observation
  • Calculated mathematically since 4th century
  • Years are numbered from Creation

In synagogue, I once overheard one man ask another, "When is Chanukah this year?" The other man smiled slyly and replied, "Same as always: the 25th of Kislev." This humorous comment makes an important point: the date of Jewish holidays does not change from year to year. Holidays are celebrated on the same day of the Jewish calendar every year, but the Jewish year is different than the civil calendar used by most of the western world, so the date shifts on the civil calendar.

Background and History

The Jewish calendar is based on three astronomical phenomena: the rotation of the Earth about its axis (a day); the revolution of the moon about the Earth (a month); and the revolution of the Earth about the sun (a year). These three phenomena are independent of each other, so there is no direct correlation between them. On average, the moon revolves around the Earth in about 29½ days. The Earth revolves around the sun in about 365¼ days, that is, about 12.4 lunar months.

The civil calendar used by most of the world has abandoned any correlation between the moon cycles and the month, arbitrarily setting the length of months to 28, 29, 30 or 31 days.

The Jewish calendar, however, coordinates all three of these astronomical phenomena. Months are either 29 or 30 days, corresponding to the 29½-day lunar cycle. Years are either 12 or 13 months, corresponding to the 12.4 month solar cycle.

The lunar month on the Jewish calendar begins when the first sliver of moon becomes visible after the dark of the moon. Note that this is different than the astronomical definition of "new moon," which would more accurately be called no moon, the darkest point of the cycle. Rosh Chodesh (first of the month) occurs a day or two after the astronomical new moon, when the moon is first visible again. In ancient times, the new months were determined by observation. When people observed the new moon, they would notify the Sanhedrin. When the Sanhedrin heard testimony from two independent, reliable eyewitnesses that they saw the moon on a certain date, the Sanhedrin would declare the Rosh Chodesh and send out messengers to tell people when the month began, so people didn't even know what month it was until after the month began! That's why Jewish holidays have an extra day: people may not have gotten word of when the month started.

The problem with strictly lunar calendars is that there are approximately 12.4 lunar months in every solar year, so a 12-month lunar calendar is about 11 days shorter than a solar year and a 13-month lunar is about 19 longer than a solar year. The months drift around the seasons on such a calendar: on a 12-month lunar calendar, the month of Nissan, which is called in Torah חֹדֶשׁ הָאָבִיב, the Month of Spring, would occur 11 days earlier each year, eventually occurring in the Winter, the Fall, the Summer, and then the Spring again. On a 13-month lunar calendar, the same thing would happen in the other direction, and faster.

To compensate for this drift, the Jewish calendar uses a 12-month lunar calendar with an extra month occasionally added. The month of Nissan occurs 11 days earlier each year for two or three years, and then jumps forward 30 days, balancing out the drift. In ancient times, this month was added by observation: the Sanhedrin observed the conditions of the weather, the crops and the livestock, and if these were not sufficiently advanced for it to be considered "spring" within the next 30 days, then the Sanhedrin inserted an additional month into the calendar to make sure that Pesach (Passover) would occur in the spring (it is commonly referred to as Chag ha-Aviv, חַג הָאָבִיב, the Festival of Spring!). Note that the traditional Chinese calendar follows the same sort of procedure, occasionally adding months to keep in sync, which is why "Chinese New Year" is always around late January or early February and Yom Kippur usually coincides with a Chinese fall festival (I'm not sure which one, but Chinatown is always very busy for the holiday the night when I break my fast!). The Islamic Hijri calendar, on the other hand, is strictly a lunar, 12-month calendar so the fasting month of Ramadan could fall in short, cool days of January (as it will in the late 2020s) or in the long, hot days of August (as it did in the early 2010s). An Islamic friend of mine once told me she preferred when it landed in January!

A year with 13 months is referred to in Hebrew as Shanah Me'uberet (שָׁנָּה מְעוּבֶּרֶת, shah-NAH m'-oo-BEH-reht), literally: a pregnant year. In English, we commonly call it a leap year. The additional month is known as Adar I, Adar Rishon (first Adar) or Adar א (the Hebrew letter Alef, being the numeral 1 in Hebrew). The extra month is inserted before the regular month of Adar (known in such years as Adar II, Adar Sheini or Adar ב). Note that Adar II is the "real" Adar, the one in which Purim is celebrated, the one in which yahrzeits for Adar are observed, the one in which a 13-year-old born in Adar becomes a Bar Mitzvah. Adar I is the "extra" Adar. If a birth or death occurs in Adar I during a leap year, it is observed during that month in a leap year and during the regular Adar in non-leap years.

In the fourth century, Hillel II established a fixed calendar based on mathematical and astronomical calculations. This calendar, still in use, standardized the length of months and the addition of months over the course of a 19 year cycle, so that the lunar calendar realigns with the solar years. Adar I is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle. The current cycle began in Jewish year 5777, which began October 3, 2016. If you are musically inclined, you may find it helpful to remember this pattern of leap years by reference to the major scale: for each whole step there are two regular years and a leap year; for each half-step there is one regular year and a leap year. This is easier to understand when you examine the keyboard illustration below and see how it relates to the leap years above.

Keyboard illustrating pattern of leap years

In addition, Yom Kippur should not fall adjacent to Shabbat, because this would cause difficulties in coordinating the fast with Shabbat, and Hoshanah Rabbah should not fall on Saturday because it would interfere with the holiday's observances. A day is added to the month of Cheshvan or subtracted from the month of Kislev of the previous year to prevent these things from happening. This process is sometimes referred to as "fixing" Rosh Hashanah. If you are interested in the details of how these calculations are performed, see The Jewish Calendar: A Closer Look.

Numbering of Jewish Years

The year number on the Jewish calendar represents the number of years since the creation of humanity, calculated by adding up the ages of people in the Bible back to the time of creation. However, this does not necessarily mean that the universe has existed for only 5700 years as we understand years. Many Orthodox Jews will readily acknowledge that the first six "days" of creation are not necessarily 24-hour days (indeed, a 24-hour day would be meaningless until the creation of the sun on the fourth "day"). For a fascinating article by a nuclear physicist shedding light on the correspondence between the Torah's age of the universe and the age ascertained by science, see The Age of the Universe: One Reality Viewed from Two Different Perspectives.

When working with dates on the secular calendar, Jews generally do not add the suffixes "A.D." and "B.C." to refer to the year. "A.D." stands for Anno Domini, which means "the year of our L-rd," and we do not believe Jesus is the L-rd. Instead, we use the abbreviations C.E. (Common or Christian Era) and B.C.E. (Before the Common Era), which are commonly used by scholars today.

Months of the Jewish Year

The Jewish calendar has the following months:

Month Name
in Hebrew
Civil Month
30 days
March - April
29 days
April - May
30 days
May - June
29 days
June - July
30 days
July - Aug
29 days
Aug - Sept
30 days
Sept - Oct
29 or 30 days
Oct - Nov
30 or 29 days
Nov - Dec
29 days
Dec - Jan
30 days
Jan - Feb
Adar I (only in leap years)
אֲדָר א
30 days
Feb - March
Adar (Adar II in leap years)
אַדָר ב
29 days
Feb - March

The length of Cheshvan and Kislev are determined by complex calculations to get the following year's Rosh Hashanah to fall on the right day. After many years of blissful ignorance, I finally sat down and worked out the mathematics involved, and I have added a page on The Jewish Calendar: A Closer Look, which may be of interest to those who want a deeper understanding or who want to write a Jewish calendar computer program. For example, I wrote a macro for Microsoft Word that puts the current Hebrew date in a document. For the rest of us, there are plenty of easily accessible computer programs that will calculate the Jewish calendar for more than a millennium to come. I have provided some links below.

Note that the number of days between Nissan and Tishri is always the same. Because of this, the time from the first major festival (Passover in Nissan) to the last major festival (Sukkot in Tishri) is always the same (177 days). Likewise the fall holidays (Rosh Hashanah, Sukkot, Shemini Atzeret / Simchat Torah) always begin two days later in the week than the previous year's Passover.

Days of the Jewish Week

The Jewish calendar doesn't have names for the days of the week, except fot Shabbat, the Sabbath, the seventh day of the week. The days of the week are simply known as first day, second day, third day, etc. Sometimes they are referred to more fully as First Day of the Sabbath, etc. Below is a list for those who are interested.

Day of the Week
in Hebrew
First Day (Sunday)
יוֺם רִשׁוֺן
Second Day (Monday)
יוֺם שֵׁינִי
Third Day (Tuesday)
יוֺם שְׁלִישִׁי
YOHM sh'lee-SHEE
Fourth Day (Wednesday)
יוֺם רְבִיעִי
YOHM r'vee-EE
Fifth Day (Thursday)
יוֺם חֲמִישִׁי
YOHM chah-mee-SHEE
Sixth Day (Friday)
יוֺם שִׁשִּֽׁי
Sabbath Day (Saturday)
יוֺם שַׁבָּת

Related Pages

Current Calendar Current Calendar
The current Jewish calendar, displaying the current three months with holidays and weekly Torah portions.
Jewish Calendar The Jewish Calendar: A Closer Look
A closer look at the mathematics behind the Jewish Calendar, explaining how Hebrew dates are calculated. Provides sample JavaScript code to show how the calculations work.

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