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The Lunar Cycle

The Jewish calendar is based on lunar cycles. 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 days—and then the thin crescent reappears, and the cycle begins again.

The entire cycle takes approximately 29½ days. 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: “Thischodesh (new moon, or month) shall be to you the head of months.” 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 Adamand 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.”

The Jewish Months and their Special Dates

Jewish Month Approximate Secular Date This Month’s Special Dates
Nissan March–April Passover
Iyar April–May Lag B’Omer
Sivan May–June Shavuot
Tammuz June–July
Menachem Av July–August Tisha B’Av
Elul August–September
Tishrei September–October The High Holidays (Rosh Hashanah andYom Kippur), Sukkot, Shmini Atzeret, andSimchat Torah
Marcheshvan October–November
Kislev November–December Chanukah
Tevet December–January Conclusion of Chanukah
Shevat January–February Tu B’Shvat
Adar February–March Purim

Sanctifying the Month

“The L‑rd spoke to Moses and to Aaron in the land of Egypt, saying, ‘Thischodesh 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.

source www.chabad.org