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The Jewish Month

The Jewish Month


The Lunar Cycle

The Jewish calendar is based on lunar cycles.1 Towards the beginning of the moon’s cycle, it appears as a thin crescent. That is the signal for a new Jewish month. The moon grows until it is full, the middle of the month, and then it begins to wane until it cannot be seen. It remains invisible for approximately two days2—and then the thin crescent reappears, and the cycle begins again.

The entire cycle takes approximately 29½ days.3 Since a month needs to consist of complete days, a month is sometimes twenty-nine days long (such a month is known as chaser, “missing”), and sometimes thirty (malei, “full”).

Knowing exactly when the month begins has always been important in Jewish practice, because the Torah schedules the Jewish festivals according to the days of the month.

The first day of the month, as well as the thirtieth day of a malei month, is called Rosh Chodesh, the “Head of the Month,” and has semi-festive status. See Why is Rosh Chodesh sometimes one day and sometimes two?

The Jewish Months

Nissan is the first month on the Jewish calendar. Before the Jews left Egypt, on the first day of the month of Nissan, G‑d told Moses and Aaron: “This chodesh (new moon, or month) shall be to you the head of months.”4 Thus the peculiarity of the Jewish calendar: the year 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. Rosh Hashanah is actually referred to in the Torah as “the first day of the seventh month.”5

The Jewish Months and their Special Dates

Jewish Month

Approximate Secular Date

This Month’s Special Dates






Lag B’Omer







Menachem Av


Tisha B’Av






The High Holidays (Rosh Hashanah and Yom Kippur), Sukkot, Shmini Atzeret, and Simchat Torah









Conclusion of Chanukah



Tu B’Shvat




Sanctifying the Month

“The L‑rd spoke to Moses and to Aaron in the land of Egypt, saying, ‘This chodesh shall be to you the head of months.’” (Exodus 12:1–2)

From the wording of this verse, “shall be to you,” the sages deduced that the responsibility of pinpointing and consecrating the chodesh, the crescent new moon, was entrusted to the leaders of our nation, the Sanhedrin, the rabbinical supreme court of every generation.

Originally, there was no fixed calendar. There was no way to determine in advance the exact day of a coming holiday or bar mitzvah, because there was no way to determine in advance when the month would begin. Each month anew, the Sanhedrin would determine whether the month would be 29 or 30 days long—depending on when the following month’s new moon was first sighted—and would sanctify the new month.


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 Hillel and his rabbinical court established the perpetual calendar which is followed today.

According to this calendar, every month of the year, except for three, has a set number of days:

  • Nissan—30
  • Iyar—29
  • Sivan—30
  • Tamuz—29
  • Menachem Av—30
  • Elul—29
  • Tishrei—30
  • Mar Cheshvan—29 or 30
  • Kislev—29 or 30
  • Tevet—29
  • Shevat—30
  • Adar—29 (in leap years, Adar I has 30 days)

Regarding the variable months of Kislev and Cheshvan, there are three options: 1) Both can be 29 days (the year is chaser), 2) both are 30 (the year is malei), or 3) Cheshvan is 29 and Kislev is 30 (the year is k’sidran, meaning these two months follow the alternating pattern of the rest of the months). Hillel also established the rules that are used to determine whether a year is chaser, malei, or k’sidran.

The rules of the perpetual calendar also ensure that the first day of Rosh Hashanah will never take place on Sunday, Wednesday or Friday.6

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

The Sanhedrin Sanctification

The following is a brief description of the procedure the Sanhedrin followed in days of yore to determine the date of the onset of a new month.

On the 30th day of every month,7 the Sanhedrin would “open for business” in a large courtyard in Jerusalem called Beit Ya’azek. Witnesses who claimed to have seen the new moon on the previous night would come to give their testimony and be cross-examined.8

The members of the Sanhedrin were well schooled in astronomy. They knew exactly when the new moon would have appeared, and where it would have been visible. Nevertheless, the sanctification of the moon depends on the crescent new moon actually being seen by two witnesses. The word “this” (in the above-quoted verse, “This month shall be to you . . .”) implies something that is actually seen.

The rabbis of the Sanhedrin would question the witnesses in the order of their arrival. They knew what the proper responses to their questions ought to be, and were thus quickly able to identify fraudulent claims. Starting with the elder of each pair, they would ask:9 “Tell us how you saw the moon:

  • In which direction was it in relation to the sun?10
  • Was it to the north or south?
  • How high in the sky did the moon appear to be?
  • In which direction were the crescent’s tips facing?
  • How wide was it?”

After they had finished questioning the first witness, they would bring in his partner and question him in similar fashion. If the two accounts corroborated, the evidence was accepted.11

That day, the thirtieth day, was now declared Rosh Chodesh of the new month. The head of the Sanhedrin would proclaim: “Mekudash!” (“Sanctified!”) and everyone would respond, “Mekudash! Mekudash!” The previous month was now retroactively determined to have had only twenty-nine days.

Publicizing the New Month

The following night (the second night of the month), huge bonfires were lit on designated mountaintops. Lookouts stationed on other mountaintops would see that a fire had been lit, and would light their own fires. This chain of communication led all the way to Babylonia, so that even very distant communities knew that the day beforehand had been declared Rosh Chodesh.

Eventually, the Sadducees12 started lighting fires on the wrong days in order to manipulate the calendar. To prevent this confusion, the fire-on-mountaintop method of communication was discontinued, and instead messengers were dispatched to Babylonia and all other far-flung Jewish settlements. This took a lot longer, a delay which had (and still has) halachic implications with regards to observance of the second day of holidays in the Diaspora. (See Why are holidays celebrated an extra day in the Diaspora?)

The 30-Day Month

If no witnesses came on the thirtieth day—either because the moon had not been “reborn” yet, or because it was not visible—then the next day, the thirty-first day, was automatically declared Rosh Chodesh, retroactively rendering the previous month a malei month.13

Members of the Sanhedrin would go to a highly visible location, where they would partake in a celebratory meal to signify the new month. No fires were lit that night. The new month is always either on the 30th or 31st day; if they hadn’t lit fires the night before, it was understood that the new month started on the 31st day.

For the spiritual spin on lunar time, see these following links:
The 29th Day
The Lunar Files


The lunar cycle which the Jewish calendar follows is called a synodic month—not to be confused with the sidereal month, the amount of time it takes for the moon to complete an orbit around the earth, which is a bit more than 27⅓ days. The synodic month is longer because after completing its orbit, the moon must move a little farther to reach the new position of the earth with respect to the sun.


For about one day before and one day after it is closest to the sun.


To be more precise, 29.5306 days.


This guarantees that Yom Kippur will not fall on a Friday or Sunday, which would result in two consecutive days when preparing food and burying the dead is prohibited; and that Hoshana Rabbah will not occur on Shabbat, which would interfere with the custom of taking the willows on this day.


If their astronomical calculations indicated that the new moon could not possibly have been seen on the previous night, the Sanhedrin would not convene on the thirtieth day.


The Talmud tells us that all the witnesses who arrived would be lavishly entertained there, in order to attract potential witnesses to travel to Jerusalem to testify.


One of the heads of the Sanhedrin, Rabban Gamliel, actually had diagrams of the various phases of the moon on a tablet mounted on the wall of his chamber. He would show these diagrams to unlearned witnesses and ask, “Did it look like this or like this?”


The new moon is visible only around the time of sunset.


Even though their testimony was no longer needed, all the other witnesses who came were questioned perfunctorily, so they should not feel that they came for nothing and would then be discouraged from coming if they ever saw the new moon again.


A sect of Jews who denied rabbinic authority, and were constantly at odds with the Sanhedrin.


On certain occasions, if the astronomical data required so, the Sanhedrin would establish Rosh Chodesh on the 30th day even in the absence of witnesses who saw the new moon. For example, suppose that the land of Israel was covered with clouds on the 30th night for several consecutive months. If the Sanhedrin would allow all these months to be malei, then several months down the line the new moon could appear on the 25th day of the month! The Sanhedrin always ensured that the new moon should never possibly appear on any night other than the 30th or 31st.

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Discussion (39)
February 14, 2017
Answer to question from Tim of St. Charles about how it adds up to 365
On the Hebrew calendar, only 12 out of every 19 years have exactly 12 months; the other 7 have 13 months each. This adds up to 235 months in every 19 years. The twelve 12-month years are 353-355 days each (off by approximately 11 days), but the seven 13-month years are 383-385 days each (off by approximately 19 days in the opposite direction). The average year is the correct length: between 365.2 and 365.25 days. (The average year on the Gregorian/secular/civil calendar, which has 97 leap years in every 400 years, is 365.2425 days.)
Camarillo, CA, USA
February 14, 2017
That's correct, the lunar year is approximately 11 days shorter than the solar year. For this reason, the Jewish calendar has a leap year every few years when we add a 13th month to align the two. Thus, in a leap year we have Adar I and Adar II - please see here for more about the Jewish leap year.
Simcha Bart for
February 13, 2017
This doesn't add up to 365. How do they account for the extra days that it takes the earth to orbit the sun. Isn't this going to throw each year off by several days?
St. Charles
November 7, 2016
You must have mistaken my question with someone else's . I asked if anyone knows the number of Sanhedrin in all of history. Thanks
Carol Ward
Portal Georgia USA
November 6, 2016
The month is completely separate from the week. A new month can begin on any day of the week and may conclude on any day of the week.

Simcha Bart for
November 5, 2016
If a new month signifies a new week what happens with the extra days? Are they extended weekends???
Cory Wolf
August 7, 2016
What is the total number of all Sanhedrin in history?
April 16, 2016
What day of the month did the 10th fall on in the Jewish Calendar?
Eddie M.
March 17, 2015
re the month of Nissan
It is read after the shachrit, morning service. You can find the text in English here . Staff
March 14, 2015
the month of nissan
During the first 13 days of the month of nissan a chapter is read every day.
Please explain at which part of the davening it is red - shaharit, mincha or maariv
With appreciation,
Since Biblical times the months and years of the Jewish calendar have been established by the cycles of the moon and the sun. Torah law prescribes that the months follow closely the course of the moon, from its birth each month to the next New Moon.
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