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Why is Rosh Chodesh sometimes one day and sometimes two?

Why is Rosh Chodesh sometimes one day and sometimes two?

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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 (Rabbinical Supreme Court) 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.

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

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 II), Iyar, Tammuz, and Elul.

The following months always have one day of Rosh Chodesh: Tishrei,1 Shevat, 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.

FOOTNOTES
1.

Not celebrated due to Rosh Hashanah.

Rabbi Naftali Silberberg resides in Brooklyn, NY, with his wife Chaya Mushka and their three children.
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Discussion (6)
February 13, 2014
Ani
There are two ways a 30 day month is determined, depending on which era of Jewish history we are discussing:

In the Temple times, there was no fixed calendar. Every month was set by way of witnesses seeing the new moon and testifying. If no witness came until after the thirtieth day, then the next day would be Rosh Chodesh, making the previous month a 30 day month. In this model, there could be multiple 30 day months one after the other, depending on when the witnesses came.

Nowadays we don't rely on witnesses seeing the new moon, rather we have a fixed calendar from year to year. The way the calendar was set up was in such a way, that (almost) every second month would have 30 days, in order to compensate for the extra 12 or so hours in each month. These days it has nothing to do with when the new moon is seen, rather it is already established in the fixed calendar.
Shaul Wolf
February 11, 2014
How the thieteeth day is formed and how two rosh chodesh are formed ?
ok i am having some confusion grasping this . Can you please explain on what basis does a hebrew month have "the 30th day" . Is it because the previous month had 29 days 12 hours , so the next month 12+12=24 hrs makes up another day (30th day) ?? Is this how a 30th day is formed in a month ?
or is it that , the 30th day in a hebrew month is taken for granted , when no moon is seen in the sky on the 30th day .

... I mean if the new crescent moon is seen on the 30th day then it becomes the first day of a new month , and no more remains the 30th day of that month , making the previous month a 29 day month . Am i Right ??
and thus the 29 day month would have only one rosh chodesh (which is its 1st day)
But if a new crescent moon is NOT seen on the 30th day in a particular month then that month is considered a full 30 day month . Am i right ?
So where does a second rosh chodesh fit in this 30 day month ?
Kindly explain . Im from Asia , trying to understand Hebrew culture . Thanks
Ani
Asia
June 20, 2007
Response to Ruvain
When the month was sanctified by the Sanhedrin, if it was a month that proved to be of thirty days (because no witnesses came), only one Korban Musaf was brought – on the thirty first day.

However, due to the possibility that in the event that witnesses would come, this day might be established retroactively as Rosh Chodesh, the masses would treat the thirtieth day as Rosh Chodesh as well, to some degree.

This was especially so on the thirtieth day of Elul, which would retroactively be Rosh Hashana, if the witnesses came. For this reason, the thirtieth day was treated as Rosh Hashana, with all the restrictions of work etc. that apply to a holiday, (besides for in regard to the sacrifices, which were dependant on if witnesses would come or not.) If no witnesses would arrive in time on the thirtieth day, the thirty-first day would be declared as Rosh Hashana as well, resulting in two days of Rosh Hashana even in Israel.
Baruch S Davidson, Chabad.org Ask the Rabbi Responder
June 12, 2007
With Sanhedrin, Celebrate One or Two Days?
A very informative article, as usual, but I would like some clarification. When a month had 30 days when there was (and G-d willing very soon will be) a Sanhedrin, did they actually celebrate both days as Rosh Chodesh with a musaf, etc., or did they only celebrate one day (either the 30th or "31st" day) depending on whether or not the 30th day was sanctified?

Thanks.
Ruvain Kudan
Albany , NY
June 10, 2007
Author's Response
Gerry Joseph,

We Jews follow a lunar calendar because of our special relationship with lunar time. For more on this subject, see "The 29th Day" (http://www.chabad.org/2764), "The Lunar Files" (http://www.chabad.org/1209), "Stony Light" (http://www.chabad.org/460543), and "The Nineteen Year Marriage" (http://www.chabad.org/1208).

It is our uniqueness which has preserved our nation for thousands of years, not our desire to be "inline" with all others.
Naftali Silberberg
June 8, 2007
The Jewish Calender
Would it not be better to change to a normal calender that is inline with the rest of the world instead of this system of following the lunar cycle which has some months having two days as head of the month? I think it would make things a lot easier Dont You? look forward to any comments you may have
Gerry Joseph
Brighton, England