In today's calendar (that is, the one established in 4119 by Hillel the Amora), months alternate between 30 days and 29 days, generally speaking. However, because of the "Arba Dechiyos" (lo adu Rosh, Molad Zakein, and two corollaries), Cheshvan and Kislev vary between 29 and 30 respectfully, both 28, or both 30 days (source: Shvili d'Rakia).

Before the calendar was set up, Beis Din established the months through witnesses (RH chapter 2). Would they ever delay the witnesses in order to keep these rules by making a month 30 days when otherwise it would be 29? Did they care about these rules?

  • 1
    I think Ramba"m Hilchot Kiddush Hachodesh discusses the concept of Bet Din rejecting witness testimony on certain occasions (or, perhaps, it had something to do with asking them extra questions so that their testimony would be rejected) the purpose being to intentionally delay Rosh Hodesh. I hav eto locate the details of this.
    – DanF
    Aug 22 '16 at 1:42
  • "the one established in 4119 by Hillel the Amora" What's that about? I'm pretty sure no one knows when exactly it was established. The Rambam just says, Until the days of Abaye and Rava...
    – MichoelR
    Mar 2 '21 at 3:17
  • @MichoelR Indeed he does. However Rav Hai Gaon and I believe the Rif as well quote this date and provenance.
    – DonielF
    Mar 3 '21 at 2:51
  • That sounds very interesting, if you have a reference.
    – MichoelR
    Mar 3 '21 at 3:50

The Rambam (Hilchos Eruvin 8:10) says that when we establish the calendar by witness testimony, then Yom Kippur can fall out on Friday or Sunday.

Even more than this, in Hilchos Kiddush HaChodesh 3:15ff, late arrivals can cause a month to be retroactively changed, even after the Chagim were already observed in Nissan and Tishrei (although for that extreme, we try hard to intimidate the witnesses to avoid).

So witness testimony has a strong place, and overrides the concerns motivating the "Arba Dechiyos".

  • I don't think that's a great proof from YK. It's possible then (Beit Din could goof in their planning) whereas it's impossible now.
    – Double AA
    Aug 23 '16 at 19:56
  • @doubleaa, hence the second point. If lo ad'u rosh, then the only time it would be possible to move rosh hashana retroactively would be when it fell out on tuesday and elul was 30 days. If there was such a major limitation, the Rambam would have mentioned it.
    – Yishai
    Aug 24 '16 at 0:00
  • @DoubleAA I actually thought it was a great proof from YK. The whole question was whether lo adu Rosh applied then, and clearly the answer is no - the Dalet and Vav part of Adu is to prevent such a scenario.
    – DonielF
    Aug 24 '16 at 4:39
  • @DonielF "clearly the answer is no" Ummm...you don't know that (from there). Rambam (there) never says it happened, just that it could happen (Beit Din could goof, or it was cloudy everywhere on 30 Elul and they were stuck, etc.; at the very least Rambam could be discussing what happened before the Lo Adu Rosh rule was designed because it's the Halakha even if it's just relevant Deorayta conceptually or in case a future greater Beit Din ends the rule).
    – Double AA
    Aug 24 '16 at 4:44
  • @DoubleAA Not to get picky or anything, but greater and bigger. And that's the point - it could happen. I think Beis Din was smart enough to avoid "goofing," as much math as they knew behind the lunar cycle. They could have foreseen such a circumstance way back in Av and delayed or hurried witness proceedings, as described in RH perek Sheini. And the Rambam does say it's only applicable when there were witnesses. I don't know, you might read the Rambam differently, but to me it sounds like a good raiyah on its own.
    – DonielF
    Aug 24 '16 at 4:59

Rambam Hiddush Hachodesh 1:6 speaks about Bais Din calculating when the new chodesh should start at deliberately not sitting or making sure to reject witnesses if they came "too early".

That is, if the Sanhedrin calculated that it should be a thirty day month, they would make sure that witnesses would not be able to testify and make it a 29 day month.

6 The [High] Court would make calculations in a manner resembling 11 the calculations of the astronomers, who know the location of the stars and their paths [in their orbits]. They would perform careful research to determine whether or not they would be able to sight the moon at the appropriate time - i.e., the thirtieth night.

If [the judges] determined that it was possible to sight [the moon], they would sit waiting for witnesses [to come and testify] throughout the entire thirtieth day. If witnesses came, and [the court] examined their testimony according to law, and verified the truth [of their statements], the court would sanctify [the new month]. If [the moon] was not sighted, and witnesses did not come,12 they would complete the thirtieth day, thus making the month full.

If, according to their calculations, [the judges] knew that it was impossible for the moon to be sighted, they would not sit [in session] on the thirtieth day, nor would they await [the arrival] of witnesses. If witnesses came, they would know that they are false witnesses, or that clouds appeared to them in a form resembling the moon, but it was not the real moon.

I seem to recall that there do appear to be references in the gemara that appear to imply that before the fixed calendar was set up there could have been occasions in which the chagim occurred on days which the fixed calendar avoids. Unfortunately, I do not have those references available now. However, this would affect only the chagim within Tishrei when the month was declared by witnesses (such as Yom Kippur and Shmini Atzeres).

Thanks to @DonielF for pointing out that the gemara in Rosh Hashana 30b states that since the time of Ezra, Elul never was more than 29 days.

Rambam Chapter 7 Halacha 1 says that Bais Din would carefully set things up so that Rosh Hashana would not fall on Sunday, Wednesday or Friday. The language used would seem to imply that Bais Din would do this even when they declared it according to witnesses, but he does not say it explicitly.

1 [Rosh Chodesh is generally instituted on the day of the conjunction. Nevertheless,] Rosh Chodesh Tishrei 1 should never be established on a Sunday, a Wednesday, or a Friday - in symbols, אד"ו - although, according to these calculations, [the conjunction for the month will occur on these days]. Instead, when the conjunction for the month of Tishrei occurs on any of these three days, Rosh Chodesh should be established on the following day. 2

What is implied? When the conjunction occurs on Sunday, Rosh Chodesh Tishrei should be established on Monday. When the conjunction occurs on Wednesday, Rosh Chodesh Tishrei should be established on Thursday. When the conjunction occurs on Friday, Rosh Chodesh Tishrei should be established on the Sabbath. 3

  • 1
    I'm not sure how the first bit helps anything. The piece in Chapter 1 just says that they wouldn't accept the witnesses any earlier than the molad. In respect to Elul, it's a Gemara in RH 19b that says that since Ezra Elul was never 30 days. And in respect to the proof in Chapter 7, to me it sounds like he's talking about the rules of the calendar, not when Beis Din accepted witnesses.
    – DonielF
    Aug 23 '16 at 23:40
  • @DonielF The way I read it was that if they calculated that the month should be thirty days, then they would not accept woinesses who would make it 29 days. The point is that they would make sure that the witnessesa did not contradict the calculation. Aug 24 '16 at 0:47

The rule that Elul is never full: Yerushalmi Rosh Hashanah 3.1

The rule that Adar adjacent to Nisan is never full: Yerushalmi Rosh Hashanah 3.1

The rule that the Day of Atonement can never occur on Friday or Sunday: Yerushalmi 'Avodah Zarah 1:1

A rule presupposing that the months from Adar to Elul all have a fixed number of days: Yerushalmi Megillah 1:2.

  • There's still the rule that the Day of Atonement can't be Tuesday
    – Double AA
    Mar 2 '21 at 2:02
  • The rule that Rosh Hashanah cannot be on Sunday was not implemented until the Geonic period. Mar 6 '21 at 0:12

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