On one hand, the Rambam says (in 2:10)

Once the court sanctifies the new month, it remains sanctified regardless of whether they erred unwittingly, they were led astray [by false witnesses], or they were forced [to sanctify it]. We are required to calculate [the dates of] the festivals based on the day that they sanctified [as the beginning of the new month].

Even if [a person] knows that [the court] erred, he is obligated to rely on them, for the matter is entrusted to them alone. The One who commanded us to observe the festivals is the One who commanded [us] to rely on them, as [implied by Leviticus 23:2]: "Which you will pronounce as days of holy convocation."

The Rambam seems to say that the "making of the new moon" of the court makes the new month (from the fact that even if they made a mistake, the new moon is sanctified).

Yet, on the other hand, the Rambam in (3:16) writes regarding witnesses who came many days after the new moon:

If the witnesses remain steadfast in their testimony, if it is compatible [according to the calculations of the court], if the witnesses are men whose character is well known, they are men of understanding, and if their testimony was scrutinized in a proper way - the moon is sanctified [retroactively]. We recalculate the dates of the month beginning from the thirtieth day [after the previous Rosh Chodesh], since the moon was sighted [on the appropriate] night.

If their sanctification is a "reason" for the Rosh Chodesh, why do they have to retract it if they realized they made a mistake?

  • The first passage you quote seems to be talking about cases where they responded to witnesses, while the second seems to be about cases where they fell back on calculation and then got witnesses later. Is that difference relevant? Does "sanctify" mean "court decreed" or "court validated testimony and decreed"? – Monica Cellio May 2 '12 at 16:44
  • @MonicaCellio I think that in the second quote, the "calculation" is just to verify that the second witnesses aren't lying (they try real hard to throw them out of court, as is mentioned in a previous Halacha there). – Shmuel Brin May 2 '12 at 22:33
  • Very interesting questions and answers. As a Karaite, I grew up observing the mo'adim in accordance with the sighting of the new moon in Israel - even when this differed from the Rabbanite calculation. It was a formative part of my Jewish identity. I even wrote about it today at my blog (ABlueThread.com). On a separate note, I have always, wondered about the "trustworthiness" of witnesses. – A Blue Thread Jan 28 '13 at 16:47
  • @MonicaCellio I think that you are correct in that the second case of retraction is where the witnesses first came after it was too late to make the month 29 days. Since it is automatically 30 days if witnesses do not come, that would appear to answer the question. – sabbahillel Nov 15 '15 at 19:17

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