This article is an excerpt from our Sefer

Buy me here or on Amazon.com

3. Days on which Rosh Chodesh will never fall:[1]

The Sages established that Rosh Hashanah, which is Rosh Chodesh Tishrei, cannot fall out on certain days of the week even if the Molad is on that day. It cannot fall on Sunday, Wednesday or Friday. In the event that the Molad falls on one of these days, Rosh Chodesh is pushed off a further day.[2] Some Rishonim[3] explain the reason for the avoidance of the above weekdays is because doing so would cause Yom Kippur to fall a day before or a day after Shabbos and they did not want there to be two consecutive days of prohibited food preparation. Likewise, they did not want Hoshana Rabbah to fall on Shabbos which would nullify the Mitzvah of Arava. The Rambam[4] however explains the reason this is done is in order to have Rosh Chodesh fall on the Cheshbon Haamiti of when the moon will be seen and not on the Molad, which is the Cheshbon Haemtzai. This decree effected that the Rosh Chodesh of every month cannot fall on certain days, as once we force Rosh Chodesh Tishrei to only fall on certain days, and the remaining months follow an automatic pattern of Malei and Chaseir, it effects that every Rosh Chodesh has only certain days on which it can fall. 

___________________________________

[1] Michaber 428:1-3; Rambam Kiddush Hachodesh 7

[2] Rambam ibid 7:1

[3] Raavad 7:7

[4] Rambam ibid 7:7