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Why is Rosh Chodesh sometimes one day and sometimes two?

by Rabbi Naftali Silberberg

  

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The Jewish calendar is based on the lunar cycle. Since a lunar month is approximately 29 days and twelve hours, we alternate months — one month is twenty-nine days and the next month is thirty. When the Sanhedrin was convened, the months were determined by witnesses who testified that they saw the crescent new-moon. The Sanhedrin would assemble on the thirtieth of each month, for perhaps witnesses would come and this day would be designated Rosh Chodesh (“Head of the Month”) of the upcoming month (rendering the previous month a 29 day month).


Since the thirtieth day of the month was always potentially Rosh Chodesh, whenever a month has thirty days, the thirtieth day is observed as Rosh Chodesh together with the next day, the first of the following month. [For more information, see How does the Jewish calendar work?]


The following months always have two days of Rosh Chodesh (the first day of the month plus the last day of the previous month): Cheshvan, Adar (and Adar I), Iyar, Tammuz,  and Elul. The following months always have one day of Rosh Chodesh: Tishrei*, Shevat ,Adar II, Nisan, Sivan, and Av. The months of Kislev and Tevet fluctuate; some years they both have one day of Rosh Chodesh, some years both have two days, and some years Kislev has one day and Tevet has two days Rosh Chodesh.


*Not celebrated. See Why don't we mention in the Rosh Hashanah prayers that the day also happens to be Rosh Chodesh?


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Shevat
The eleventh month on the Jewish calendar, normally corresponding to January-February.
Tishrei
The seventh month of the Jewish calendar. This month, which arrives in early autumn, has more holidays than any other month: Rosh Hashanah, Yom Kippur, Sukkot and Simchat Torah.
Sanhedrin
The Jewish Supreme Court. The court would convene in a designated chamber in the Holy Temple, and was comprised of 71 of the greatest scholars of the time. Continued after the destruction of the Temples, but was dissolved in the 5th century when due to Roman persecution the seat of Torah scholarship relocated from Israel to Babylon.
Adar
The twelfth month on the Jewish calendar. This month (which falls out approx. February-March), is the most joyous month on the calendar due to the holiday of Purim which is on the 14th and 15th of this month.
Tevet
The tenth month on the Jewish calendar. Falls out in mid-winter.
Rosh Chodesh
The "Head of the Month," Rosh Chodesh is observed the first day of every Jewish month. If the previous month had 30 days, then the last day of the previous month is also observed; hence a two-day Rosh Chodesh. Rosh Chodesh is a semi-holiday, marked by Torah-reading and special prayers.
Iyar
The second month on the Jewish calendar, normally corresponding to April-May. The 18th of this month is the holiday of Lag b'Omer.
Sivan
The third month on the Jewish calendar, normally corresponding to May-June. This month features the holiday of Shavuot.
Kislev
The ninth month on the Jewish calendar, normally corresponding to November-December. The holiday of Chanukah starts on the 25th of this month.
Elul
The 6th month on the Jewish calendar, normally corresponding to August-September. This is the month which precedes Tishrei, the month of the High Holidays, and is a month of introspection and repentance.
Cheshvan
The eighth month of the Jewish calendar, normally corresponding to October-November.
Av
The fifth month of the Jewish calendar, normally corresponding to July-August. The saddest month of the year due to the destruction of the Temples, and the many other tragedies which befell the Jews in this month.
Tammuz
The fourth month on the Jewish calendar, normally corresponding to June-July.