1,248. Until When Testimony Was Accepted

Hilchos Kiddush HaChodesh 3:5

Originally, the court would accept testimony about the new moon the entire 30th day. It once happened that the witnesses were delayed and didn’t arrive until the evening. This created confusion in the Temple so that the kohanim didn’t know what to do. If they offered the afternoon sacrifice and witnesses arrived, it would no longer be possible to offer the Rosh Chodesh musaf sacrifices. The Sanhedrin therefore enacted that testimony about the moon would be accepted only until the time of the afternoon offering so that there would be enough time during the day to offer the musaf, the afternoon sacrifice and their libations.

Hilchos Kiddush HaChodesh 3:6

If the time of the afternoon sacrifice arrived without witnesses appearing, it was offered. If witnesses came after this, both that day and the next were observed as holidays. The musaf sacrifice was only offered on the second day because the new month could not be sanctified after the time of the afternoon offering. After the Temple was destroyed, Rabbi Yochanan ben Zakkai and his court instituted that the court would accept testimony about the moon for the entire day. Even if witnesses came at the end of the 30th day before sunset, their testimony was accepted and only the 30th day was observed as a holiday.