1,264. Only the Sanhedrin Could Set the Calendar

Hilchos Kiddush HaChodesh 5:1

Everything we said previously about sanctifying the new month because of seeing the moon, and about establishing a leap year to adjust the lunar and solar calendars or out of necessity, only applies to the Sanhedrin in Israel. Only they, or a court of judges possessing the original ordination that convenes in Israel and was granted authority by the Sanhedrin, may perform these actions. This is derived from Exodus 12:2, in which God told Moses and Aaron, “This month will be the first of months for you.” Our oral tradition, transmitted teacher to student all the way back to Moses, explains that this duty was entrusted to Moses and the communal authorities who would succeed him. When there is no Sanhedrin in Israel, we only set the months and establish leap years based upon our current, fixed calendar.

Hilchos Kiddush HaChodesh 5:2

This idea is a law communicated to Moses at Sinai, that when there is a Sanhedrin, the months are set based upon witnesses seeing the moon, and when there is no Sanhedrin, the months are set based upon our fixed calendar. Under current circumstances, witnesses seeing the moon is of no consequence. When the fixed calendar is followed, sometimes Rosh Chodesh will be a day on which the moon is seen, and sometimes the moon will be seen the day before or the day after. It is highly uncommon for Rosh Chodesh to be the day after the moon’s appearance; this only happens in lands that lie to the west of Israel.