What is the "Olas ha'Chodesh" to which the Pasuk mentions here?
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 first it needs to inform 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.
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.