1,281. Why Rosh Hashana Isn't the Day of the Molad

Hilchos Kiddush HaChodesh 7:7

The reason that Rosh Hashana is not set for the same day when the molad falls on Sunday, Wednesday or Friday is because the calculations only determine the conjunction of the sun and the moon based upon their average rates of motion; they do not necessarily reflect their actual positions, as has been explained. The Sages therefore established days on which to set Rosh Chodesh and days on which to postpone it in order to determine the day of the actual conjunction. Therefore, if we calculate the molad for Tuesday, it is set as Rosh Chodesh; if on Wednesday, it is postponed; if on Thursday, it is set; if on Friday, it is postponed; if on Shabbos, it is set; if on Sunday, it is postponed; if on Monday, it is set.

Hilchos Kiddush HaChodesh 7:8

The fact that the calculations only represent the average rate of motion is also the reason for the other four things that postpone Rosh Chodesh. The proof of this is that sometimes the molad is calculated to occur on a Tuesday but Rosh Chodesh is postponed until Thursday. This is so even though the moon will not yet be visible on Thursday night, or even on Friday night. We see from this that the actual molad did not occur until Thursday.