Why for certain months of the year do we celebrate the festival of Rosh Chodesh for two days while on other Roshei Chadashim we have one day of Rosh Chodesh?

4 Answers 4


Tur (Orach Chaim 427) explains as follows:

A lunar month is 29-1/2 days plus some extra chalakim. Since a month has to have an integral number of days, the months (usually) alternate between 30 and 29. The 30th day, then, is always going to have to be Rosh Chodesh:

  • If the month was 29 days, then the 30th day is the first day of the next month, and is Rosh Chodesh by definition.

  • If the month is 30 days, then part of the 30th day really already belongs to the next month (because, as above, the month should really have ended midway through that day), and therefore that day is celebrated as Rosh Chodesh.

In the latter case, then, you're going to have two days of Rosh Chodesh, since after all the 30th day belongs to the previous month, and so the first day of the next month is again Rosh Chodesh by definition.

  • 1
    -1. In the last months you can see that the molad is couple of days before rosh chodesh, so how does it suits your answer?
    – jutky
    Nov 27, 2011 at 21:37
  • @jutky, I'm not sure that the actual time of the molad makes a difference for this purpose. Regardless of when that occurs, the fact is that a month consists of more than 29 days. (I suppose it's possible that the Tur's expression הואיל ומקצתו ר"ח means, as in Jon's answer, that part of the day has already been observed provisionally as Rosh Chodesh while waiting for the witnesses to show up; but then עיקר חסר מן הספר, since Tur hasn't said anything about witnesses.)
    – Alex
    Nov 28, 2011 at 16:06
  • @jutky Are you saying the Tur is wrong?
    – Double AA
    Apr 20, 2015 at 18:57
  • The Tur is talking about when Hillel instituted a new fixed lunar calendar, where you have alternating months i.e 30/29. When the Bais Hamikdash was around the reason why they observed two days of Rosh Chodesh was entirely different.
    – user13749
    Nov 30, 2016 at 20:15
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    Does the Tur actually say this is the reason we practice two days Rosh Chodesh, or simply why some months are 29 days and others are 30? Because having 30 days in the month is not a reason to keep the 30th day as Rosh Chodesh.
    – chortkov2
    Jan 14, 2020 at 17:32

I disagree with the answer above. The month is not decided by the moon cycles, but by Witness testimony, so any astro science becomes somewhat irrelevant. For example if there are witnesses accepted when the moon is not new yet, we go after the Witnesses, not the actual placement of the moon. So I thinks its wrong to say: then part of the 30th day really already belongs to the next month

2 days comes from the same problem that creates 2 days Rosh HaShanna in Israel (although RH in Israel is fundamentally different as the 2nd day of the month is celebrated as a festival, which could never happen.) You will see that 2 days Rosh Codesh is only when the previous month is 30 days. If it is 29 days, the rosh chodesh is only 1 day.

Since a month has to be 29 or 30 - As explained in the previous answer the 30th day is either the 30th day of month A, or day 1 of month B. Day 30 is the only unknown in all this. Witnesses will either come on the 30th and turn it into the 1st, or they wont come, and the 31st will have to become 1st of month B.

Witnesses have to come in the daytime, so even if they come at the earliest time possible, the 30th has already had 12 hours before it turns into the 1st.

So we keep every 30th as a Rosh Chodesh in case the Witnesses come.

According to what I am saying, if the Mashicah comes on the 30th of the month, and the court is established in time, and witnesses come, they can technically turn that day into the 1st. So we have to keep it just in case.

(Similarly by Rosh hashanna - its not a delay in witnesses but a fundamental problem that what day it is today will not be know till some point in the future that creates the 2 days in Israel. And we now keep the 2nd day on day 2(not 30 & 1) as a remembrance of this)

  • 1
    Hello @Jon and welcome to Judaism.SE! Re "I disagree with the answer above": Are you fundamentally disagreeing with the Tur's assertion that there is a fixed period of time between cycles of the moon? If so, are you basing your disagreement on a printed source or is it your own?
    – WAF
    Nov 27, 2011 at 14:55
  • +1. @WAF: there is no problem with Tur, indeed there is a fixed period between moon cycles. 2 days is because of witnesses.
    – jutky
    Nov 27, 2011 at 21:36

I wondered about this for many years, until I finally found that the (13th century Italian) Shiboley HaLeket discusses the question explicitly. See the Shiboley HaLeket in Siman 168, who in turn quotes the (fellow 13th century Italian rabbi) Rabeinu Yeshaya which can be found in Rabeinu Yeshaya's Teshuvos, number 32.

Loosely translated and highly paraphrased, what Rabeinu Yeshaya says is that the reason Rosh Chodesh is two days only in cases of 29 day months is because there are two days that each have aspects of being Rosh Chodesh. Day "30" becomes day 1 of the incoming month, and hence is the day of actual kidush hachodesh. Day "31" which is now officially day 2 of the incoming month, is considered Day 1 for matters of counting the month and for legal documentation(שטרות). Since both days have aspects of being the first of the month in that situation, we observe two days.


A little bit of background for this. In the days of the Temple, the new moon was actually CORROBORATED by witnesses. They were not the ones who declared whether a new moon was sighted or not. That task fell to the court that sat at the Temple, whose task it was to ensure that the witnesseses actually saw a new moon in its proper place. However, in some exceptional cirucumstances, a court can proclaim unilaterally that a month has an extra day in it or not. There is a story of Rabbi Akiva proclaiming that an extra day should be added to a month - this, while he was locked up in prison (Yevamot 104).

Having said all that, in our day the Jewish calendar was set in its way by Hillel II. Realizing that being able to proclaim a new moon by a court of Rabbis given proper semicha (not the kind given nowadays, but given from teacher to student from Moses to the time of the second Temple) would soon be impossible, he and a court unilaterally proclaimed the new moon for each month based on calculations (including ensuring that holidays such as Yom Kippur would not fall on a Friday or Sunday). In this case, some months have 29 days and some have 30 days. Even though we can nowadays compute exactly when the new moon is, we still use the calendar based on the idea that only a Jewish court of equal or greater knowledge can overrule a ruling from a previous Jewish court. Once we get to the time of the Messiah, then we will go back to having witnesses telling us when the moon was visible or not.

  • So why are there 2 days of Rosh Chodesh? As you say "we can compute exactly", why do we celebrate the 30th as Rosh Chodesh? We know its not...
    – user1040
    Nov 28, 2011 at 13:12
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    This has to do with keeping tradition alive, more than anything else. In the days of the Temple, a second day was added to allow for the possibility that the first OR second day was Rosh Chodesh (for those living outside the Jerusalem area who couldn't be contacted within one day). This was important for holidays such as Pesach, when everyone needed to be on the same page. Although our calendar is based on computations, it tries and retain that same tradition - it's the same reason Jews outside Israel have two days of Pesach instead of one. Nov 28, 2011 at 13:42
  • If it "has to do with keeping tradition alive, more than anything else" can you edit your answer to reflect this? Also - do you then disagree with my point that depending when the Mashiach comes that the 30th cant be made into the 1st. If not - then we have to keep the 30th as Rosh Chodesh whether we keep traditions or not.
    – user1040
    Nov 30, 2011 at 13:02

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