1,245. If the New Month is Declared in Error

Hilchos Kiddush HaChodesh 2:9

If the members of the court personally observed the moon at the end of the 29th day before a star emerges on the night of the 30th, they may proclaim, "It is sanctified, it is sanctified” because it’s still day. If they see the moon on the night of the 30th after two stars have appeared, they proceed as follows: the next day, two more judges join together with one of the three to form a court. The other two then testify before this court, which then sanctifies the month.

Hilchos Kiddush HaChodesh 2:10

Once the court has sanctified the month, it remains sanctified even if the judges made a mistake, were misled by false witnesses or were coerced into declaring the new month. We must calculate the holidays based on the day that the court sanctifies. Even if someone knows that the court has made a mistake, he is obligated to rely on them because this is their exclusive purview. The One Who commanded us to observe the holidays is also the One Who commanded us to rely on the court when it comes to setting the dates, as we see from Leviticus 23:2, “that you will proclaim as days of holy gathering.”