1. It Is Used to Calculate Holidays, Bar Mitzvahs, and Yahrzeits

The Jewish calendar is the structure upon which all Jewish holidays are based. The High Holidays, Sukkot, Chanukah, Purim, Passover and Shavuot are always celebrated on their specific dates on the Jewish calendar. (For example, Rosh Hashanah is always celebrated on 1–2 Tishrei, and Passover always begins on 15 Nisan.)

Other notable occasions that follow the Jewish calendar are birthdays, yahrzeits, bar mitzvahs and bat mitzvahs.

Read: How to Celebrate Your Jewish Birthday and Calculate your Jewish birthday

2. It Was Given to Moses at the Exodus

Exactly two weeks before the Exodus from Egypt, G‑d told Moses and Aaron: “This month [Nisan] shall be for you the head of the months,”1 setting into motion the Jewish calendar and its unique format. In fact, this was the very first commandment G‑d gave to Moses.

Read: 12 Facts About the Month of Nisan Every Jew Should Know

3. The Greeks Tried to Abolish It

The Chanukah story begins with the persecution of the Jews by the Syrian Greeks. Recognizing the crucial role of the calendar in Jewish life, the Greek king Antiochus decreed that the Sanhedrin was forbidden to sanctify new months (see below). Led by the Maccabees, the Jews defied the royal edict, culminating in a victory over their oppressors and the creation of a new Jewish holiday in our calendar.

Read: Why Couldn’t the Jews and Greeks Just Get Along?

4. It Follows the Lunar Cycle, But Is Still Aligned with the Seasons

Unlike the Gregorian calendar, which follows the solar cycle (of about 365.25 days), the Jewish calendar follows the lunar cycle, which means that the year is comprised of 12 lunar months (of approximately 29.5 days each).

Nevertheless, the Jewish calendar is not solely lunar. Due to the 11-day discrepancy between the solar year (365 days) and the lunar year (29.5 × 12 = 354), lunar calendar dates are not tied to the seasons. If a certain day occurs this year in the spring, in a few years it will regress to the winter, and so on.

On the Jewish calendar, referred to as a lunisolar calendar, the dates are aligned with the seasons. For example, Passover must be celebrated in the spring. In order to prevent a regression, every two or three years a thirteenth month is added (more on that below).

Read: Introduction to the Jewish Calendar

5. The Names of the Months Came from Babylonia

Scripture generally describes the months based on their place in the calendar—e.g., third month, fourth month, and so on. The Hebrew names of the months as we know them today were brought to Israel from Babylonia at the onset of the second Jewish commonwealth, approximately 350 BCE.

Read: Why Babylonian Names for Jewish Months?

Here is a list of the Jewish months and their important dates:

Jewish Month Approximate Secular Date This Month’s Special Dates
Nisan March–April Passover
Iyar April–May Lag B’Omer
Sivan May–June Shavuot
Tammuz June–July Fast of 17 Tammuz
Menachem Av (also known as Av) July–August Fast of 9 Av
Elul August–September
Tishrei September–October The High Holidays (Rosh Hashanah and Yom Kippur), Sukkot, Shemini Atzeret and Simchat Torah
Marcheshvan (also known as Cheshvan) October–November
Kislev November–December Chanukah
Tevet December–January Conclusion of Chanukah and 10 Tevet fast
Shevat January–February Tu B’Shevat
Adar February–March Purim

6. Months Are Either 29 or 30 Days

In the Gregorian calendar, most months are either 30 or 31 days (because 365 ÷ 12 = 30.4). In the Jewish calendar, since the lunar cycle is about 29.5 days, all months are either 29 days (known as “missing” months) or 30 days (known as “complete” months).

Most months have a set number of days (Nisan—30, Iyar—29, Sivan—30, Tammuz—29, and so on). There are two exceptions: Marcheshvan and Kislev can be either 29 or 30 days (see below).

7. Leap Years Have Thirteen Months

In the Gregorian calendar, every four years an extra day is added, creating a leap year—a year with 366 days instead of 365. In the Jewish calendar, however, leap years have an additional month.

The Torah specifies that Passover must be celebrated in the spring2 and Sukkot during autumn.3 This poses a problem, as the lunar year is eleven days short of the solar year, and any given date will potentially regress from one season to the next.

In order for the festivals to retain their positions relative to the seasons, an adjustment must be made to enable the lunar calendar to maintain harmony with the solar cycle. To do so, years are grouped into 19-year cycles. In the 3rd, 6th, 8th, 11th, 14th, 17th and 19th year of every 19-year cycle, another month (Adar) is added. Such a year is called “shanah m’uberet,” literally “a pregnant year.”

Read: 13 Facts About Jewish Leap Years

8. A Year Can Be Between 353 and 385 Days Long

The months of Marcheshvan and Kislev are variable. On any given year they can both be 29 days; they can both be 30 days; or Marcheshvan can be 29 days while Kislev is 30.

Based on this, any given year can contain either 353, 354, or 355 days (or in a leap year: 383, 384, or 385 days). When both months are 29 days, the year is known as chaseirah (missing); when both are 30, the year is shleimah (complete); and when Marcheshvan is 29 days and Kislev is 30, the year is k’sidrah (regular, meaning these two months follow the alternating pattern of the rest of the months).

Read: The Jewish Calendar Year

9. There Are Four New Year Days

In addition to Rosh Hashanah (1 Tishrei) being the first day of the year, there are three more dates that are considered “New Year days” with regard to specific laws and rituals: 1 Nisan, 1 Elul, and 15 Shevat. Of the three, the most well-known is 15 Shevat, also known as Tu B’Shevat—the New Year for Trees.

Read: Tu Bishvat: What and How

10. The First Month Is Halfway Through 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, G‑d told Moses and Aaron: “This month shall be for you the head of the months.”4 Thus the peculiarity of the Jewish calendar: The year begins on Rosh Hashanah, the first day of the month of Tishrei, but Tishrei is not the first month. Rosh Hashanah is actually referred to in the Torah as “the first day of the seventh month.”5

Read: Our Other Head

11. New Months Were Originally Determined Based on Lunar Sightings

Originally there was no fixed calendar. Each month, the Sanhedrin—the rabbinical supreme court—would determine whether that month would contain 29 or 30 days, depending on when the following month’s new moon was first sighted.

On the 30th day of every month, the Sanhedrin began accepting witnesses who claimed they had spotted the new moon the previous evening. If the witnesses would pass the court’s rigorous interrogation, the Sanhedrin would “sanctify” the new month, proclaiming that day the first of the month. The previous month was now retroactively determined to have had only 29 days.

If no witnesses came on the thirtieth day, then the next day, the thirty-first day, was automatically declared the first day of the new month, retroactively rendering the previous month a “complete” month of 30 days.

Read: The Jewish Month

12. The Calendar We Use Today Was Established in the 4th Century CE

In the 4th century CE the sage Hillel II foresaw the disbandment of the Sanhedrin, and understood that we would no longer be able to follow a Sanhedrin-based calendar. So he and his rabbinical court established the perpetual calendar which we follow today.

When Hillel established the perpetual calendar, he sanctified every new month until Moshiach will come and reestablish the Sanhedrin.

13. Outside Israel, Holidays Have an Extra Day

Originally, when there was no fixed calendar, there was no way to determine the exact day of a coming festival in advance. This was because every festival falls on a particular day of a month, and the month would begin only when the new moon of that month was sighted.

Once the Sanhedrin had determined that a new moon had been sighted, messengers were dispatched to Babylonia and other far-flung Jewish settlements to relay this information to them. Since news traveled a lot slower in those days, many communities outside of Israel would not know when the new month had begun in time to celebrate the festival on the proper day. To cover both possibilities, they would celebrate every holiday for two days: the day the holiday would be if the previous month had 29 days, and the day it would be if the previous month had 30 days.

Read: Why Do We Still Celebrate Holidays for Two Days in the Diaspora?

14. Every Week Has a Unique Name

In addition to the names of the twelve (or thirteen) months, each week of the year has its own name, based on the Torah portion read in the synagogue that Shabbat.

Mystical sources indicate that we should attempt to gain insight and understanding into the week’s events by delving into the weekly Torah portion. The week’s Torah reading, known as the Parshah, has special relevance to everything that happens during that week.

Explore this week’s Torah portion

15. Certain Time-Related Laws Follow the Solar Cycle

Although the Jewish calendar follows the lunar cycle, certain time-related laws and customs follow the civil calendar, which follows the solar cycle. Among them:

  • The Amidah prayer includes a request for rain in the winter months (“vetein tal umatar livrachah”). In a regular year we start saying this prayer on the night of December 4 (preceding December 5), and in the year before a civil leap year (2023, 2027, 2031, 2035), on the night of December 5 (preceding December 6).6
  • Read: Why Is the Prayer for Rain Based on the Civil Calendar?

  • When the sun returns to the same position that it occupied at the time of its creation, at the same time of the week, we recite a special blessing to G‑d known as Birkat Hachamah. This mitzvah is performed once every 28 years, on April 8 (it was last performed in 2009, and will be performed again in 2037, 2065 and 2093).7
  • Read: Calculating the Date of Birkat Hachamah

16. The First Day of Each Month Is a Minor Holiday

The first day of each month (and sometimes, the last day of the previous month—see below) is known as Rosh Chodesh (lit., “head of the month”). Special prayers are added to the daily services, and we wish each other “chodesh tov,” a good month.

Since the 30th day of the month was always potentially Rosh Chodesh (see above), whenever a month has 30 days, the last day is observed as Rosh Chodesh together with the first of the following month.

However, if a month has only 29 days, then the Rosh Chodesh of the following month will be only one day—the first of the month.

Read: Why Is Rosh Chodesh Sometimes One Day and Sometimes Two?

17. The First Year Was Only Five Days Long

The years of the Jewish calendar are calculated from the creation of the world. Hence, in 2020, it is presently 5780 years since Creation.

However, Rosh Hashanah, the Jewish New Year, is celebrated on the day Adam was created, which was really the sixth day of creation. Creation itself began five days earlier, on the 25th day of the month of Elul.

When we give the number of 5780, we actually mean that it is presently 5779 years and five days since Creation, as those initial five days are considered Year 1, and Adam’s creation marked the beginning of Year 2.

Read: Which Year Was the Second Temple Destroyed?

The Jewish calendar is essential to the structure of Jewish life. Visit our online calendar for a curated collection of Jewish history, laws and customs, daily study, and a thought for each day of the year.