1,253. Sanctifying the Month Retroactively

Hilchos Kiddush HaChodesh 3:15

If the court sat in session for the entire 30th day but witnesses didn’t arrive, they would get up early in the morning and declare the month full, as was previously explained (in halacha 3:7). If witnesses arrive from a distant place four or five days later and testify that they had seen the moon in its proper time on the 30th night – or even if they just come at the end of the month – then they are interrogated in an intimidating fashion in order to confuse them. They are examined very thoroughly with precise questions about their testimony. This is because the court is attempting not to sanctify the month because it was already pronounced full.

Hilchos Kiddush HaChodesh 3:16

If the witnesses stand firm in their testimony and it consistent with the court’s astronomical expectations, and if the witnesses are known to be people of good character and understanding, and if their testimony was properly scrutinized, then the month is sanctified retroactively. The dates of the month are recalculated from the 30th day because the moon was seen in its proper time.