Jewish Calendar : Hebrew Months
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
⁰Jewish Month ; Approximate Secular Date ;This Month’s Special Dates
¹ Nissan ; March–April ; Passover
² Iyar ; April–May ; Lag B’Omer
³ Sivan ; May–June ; Shavuot
⁴ Tammuz ; June–July ; (nil)
⁵ Menachem Av ; July–August ; Tisha B’Av
⁶ Elul ; August–September ; (nil)
⁷ Tishrei ; September–October ; The High Holidays (Rosh Hashanah and Yom Kippur), Sukkot, Shmini Atzeret, and Simchat Torah
⁸ Marcheshvan ; October–November ; ( nil )
⁹ Kislev ; November–December ; Chanukah
¹⁰ Tevet ; December–January ; Conclusion of Chanukah
¹¹ Shevat ; January–February ; Tu B’Shvat
¹² Adar ; February–March ; Purim
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.
Nowadays
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 Samaritans¹² 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.
Footnotes
1.
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.
2.
For about one day before and one day after it is closest to the sun.
3.
To be more precise, 29.5306 days.
4.
Exodus 12:2. “This month is to be for you the first month, the first month of your year.
Exodus 12:2 establishes the month of Abib (later known as Nisan) as the first month of the year for the Israelites, marking a significant new beginning as they prepare to leave Egypt. This change in the calendar symbolizes God's intervention and the start of their identity as a free people.
5.
Leviticus 23:24.
6.
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.
7.
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.
8.
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.
9.
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?”
10.
The new moon is visible only around the time of sunset.
11.
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.
12.
A sect of Jews who denied rabbinic authority, and were constantly at odds with the Sanhedrin.
13.
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.
Why Do We Still Celebrate Holidays for Two Days in the Diaspora?
Is the Yom Tov Sheini an anachronism?
Question:
I understand that in ancient times the rabbis decreed that Jews in the Diaspora should celebrate holidays for two days, because of some confusion about the correct day to celebrate. Nowadays, however, we have a fixed calendar, so why do we still celebrate two days in the Diaspora?
Answer:
Good question. Indeed, we could ask why the custom of celebrating two days didn’t stop almost 2,000 years ago, when the Jews started using a fixed calendar as opposed to determining the months based on observations of the new moon.
In Ancient Times
Before getting to the crux of the question, Let's first start with a brief overview of how Jewish holiday dates work:
Whereas Shabbat happens every seven days without any input from us, G‑d commanded the children of Israel to fix the dates of the months and years.¹ This means that the power to determine when the holidays fall was given to Moses and his successors—the rabbinical courts.
Originally, there was no fixed calendar. Each month, the Sanhedrin (supreme court) in Jerusalem would determine whether the previous month had been 29 or 30 days long, depending on when the new moon of the following month was first sighted.
There was no way to determine the exact day of a coming festival (Yom Tov) in advance, because every festival falls on a particular day in 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 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 to even more distant communities. 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 who denied rabbinic authority and were constantly at odds with the Jews, started lighting fires on the wrong days in order to manipulate the calendar.
To prevent confusion, the fire-on-mountaintop method of communication was discontinued, and instead messengers were dispatched to Babylon and other farflung Jewish settlements.
Since news traveled a lot slower that way, distant communities would not know when Rosh Chodesh (the “Head of the Month”) 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.
The Custom of Your Forefathers
The fixed calendar began being used in the 4th century CE, and so everyone knew in advance when Rosh Chodesh and the festivals would occur. However, the Talmud explains that although the doubt about the calendar is no longer relevant, we are still bound according to rabbinic law to observe a second day, in case a doubtful situation were to arise again. In the words of the Talmud, “The sages sent [word] to the exiles, ‘Be careful to keep the customs of your forefathers, and keep two days of the festival, for someday the government may promulgate a decree, and you will come to err.’”²
However, the question remains. A simple reading of the Talmudic text cited above seems to indicate that the reason to still keep two days is merely as a precaution in case we are not able to learn the Torah and the proper way to calculate the new month. However, nowadays, with the advent of modern technology and computers, we have calendars stretching for hundreds of years into the future, so this would hardly seem to be a concern.
An Enactment of the Prophets
Rabbi Hai Gaon was the undisputed authority on Jewish law in the early 11th century. He writes that the requirement that one should keep a second day of Yom Tov outside of Israel really stems from the days of the prophets, and perhaps even from the days of Yehoshua (Joshua) ben Nun.
Based on this, he explains the somewhat curious wording of the Talmud cited earlier: “Be careful to keep the customs of your forefathers and keep two days of Yom Tov.” Why not state simply that we should keep two days lest there develop some doubt as to the correct day?
However, there is a general rule that once a rabbinical enactment has been made by the Sanhedrin and accepted by the entire Jewish people, the enactment can be absolved only by a court that is similar or greater in number and stature to the one that made the enactment. While one would be hard-pressed to find a court that could compare to even a regular Sanhedrin, it would be impossible to find a court with the same stature as the prophets, who were divinely inspired. Thus the Talmud warns that we should “be careful to keep the customs of your forefathers,” as the custom of keeping two days of Yom Tov in the Diaspora isn’t just about satisfying a doubt, but about adhering to an enactment instituted by the prophets.
Rabbi Hai Gaon concludes that like many of the enactments of the prophets, we often do not know the real reason or “secret” behind their enactment.³
The Holiness of the Land of Israel
According to the teachings of the inner wisdom of Torah, there is a deeper reason—perhaps even the “secret” of the prophets to which Rabbi Hai Gaon alludes—for keeping two days of Yom Tov in the Diaspora.
The Tzemach Tzedek, the third Lubavitcher rebbe, expounding on what Kabbalists such as Rabbi Moshe Cordovero and others write, explains that since holiness is more revealed in the Land of Israel, the festivals can be revealed and received there in one day, as the Torah commands. However, those in the Diaspora are farther away from the revelation of holiness, and therefore, in order to absorb the spiritual emanations of the festivals, two days are required, as the rabbis ordained.
The Tzemach Tzedek explains this phenomenon using the metaphor of a bright torch: When held close to an object, the light is strong and concentrated on a small area; but when the torch illuminates an object from a distance, its light is weakened and dispersed over a large area. Thus, the light of the festivals is revealed in the Land of Israel in one concentrated and focused day, while in the Diaspora the light of the festivals is weaker and spread out over two days.⁴
Perhaps it is for this reason⁵ that the Lubavitcher Rebbe,⁶ as well as others,⁷ contends that even in the days of Moshiach, when the Holy Temple is rebuilt and the Sanhedrin is reestablished, those outside of Israel proper will continue to celebrate Yom Tov for two days. After all, although the whole world will be elevated, the Land of Israel will be elevated with an even greater degree of revelation. May this be speedily in our days!
Please note: Regarding the law for travelers to or from Israel, although some authorities say that you should keep Yom Tov for one day if you are in Israel and two days if you are elsewhere, most authorities rule that wherever you are, you keep Yom Tov for the number of days it is kept in the country of your permanent residence. One should consult with one’s own rabbi before traveling.
Footnotes
1.
Exodus 12:1.
2.
Talmud, Beitzah 4b.
3.
Otzar HaGeonim, Masechet Yom Tov (Beitzah) 4b.
4.
Derech Mitzvotecha 198a.
5.
See Yemot Hamoshiach Bahalachah 1:46.
6.
Torat Menachem 5749, vol. 1, p. 227 (talk during the day of Simchat Torah).
7.
See for example Derashot Chatam Sofer, vol. 2, p. 272b.
