1,278. When Rosh Chodesh Tishrei Cannot Fall

Hilchos Kiddush HaChodesh 7:1

Under the current fixed calendar, Rosh Chodesh Tishrei is never set to fall on Sunday, Wednesday or Friday (abbreviated 1, 4, 6) even if that’s what the calculations call for. If the molad for Tishrei falls on any of these days, Rosh Chodesh is set for the following day. Therefore, if the molad falls on Sunday, Rosh Chodesh Tishrei is set for Monday. If the molad falls on Wednesday, Rosh Chodesh Tishrei is set for Thursday. If the molad falls on Friday, Rosh Chodesh Tishrei is set for Shabbos.

Hilchos Kiddush HaChodesh 7:2

Similarly, if the molad for Tishrei falls at noon or after noon, Rosh Chodesh is set for the following day. Therefore, if the molad falls on Monday at six hours past daybreak or later, Rosh Chodesh is set for Tuesday. If the molad occurs before noon - even by a single chalak - Rosh Chodesh is set for that day, so long as it isn’t a Sunday, Wednesday or Friday.