What is the "Olas ha'Chodesh"?
Rashi: It is the Olah of the Musaf of Rosh Chodesh, which coincides with Rosh Hashanah. 1
See above, 28:11.
Why does the Torah mention the Olas ha'Chodesh on Rosh Hashanah, but not the Chatas ha'Chodesh?
Ramban: Because having informed us that the Olah mentioned here is not the same one that is mentioned on Rosh Chodesh, 1 it is obvious that the Chatas is not the same one either.
Hadar Zekenim (citing a Midrash) and Riva: Because, whereas the Kaparah for all Olos is the same - for [transgressing] an Asei, or a La'av ha'Nitak la'Asei, the Chata?os of Rosh Chodesh and Rosh Hashnah are not. 2
Ramban: Since we would otherwise have thought that the Olah mentioned on Rosh Chodesh is confined to the other eleven months, but is different on Rosh Hashanah.
The Chatas of Rosh Chodesh atones for [Aveiros of Tum'ah] of which the owner was not aware, either at the beginning or at the end, and of Rosh Hashanah, when there is knowledge only at the end.
