Hebrew calendar
Postponing Rosh Hashanah so that it does not start on Sunday, Wednesday or Friday. This rule prevents Yom Kippur from being the day before or after Shabbat. It also prevents Hoshana Rabba, the seventh day of Sukkot, from being on Shabbat.Once Rosh Hashanah for this year is calculated, the Rosh Hashanah for next year is calculated, starting with the time either 12 or 13 molad periods in the future.