Q. Why do we blow the shofar on Rosh HaShanah during Musaf, not during Shacharit?A. Since there is a principle, Z’rizim makdimim l’mitzvot – “One should be eager and perform the commandments early” (Pes. 4a), it would have been logical to sound the shofar during the morning service.
The medieval liturgy, Machzor Vitry (chapter 316), says: “Our enemies (the Romans?) once decreed against the blowing of the shofar (did they think it was a Jewish call to arms against the regime?) and they would watch for it until the end of the Shacharit; for this reason it was decided to transfer the sounding of the shofar to Musaf.
“However, when the decree was abolished, they did not do away with this enactment and based it on a passage in the P’sikta, ‘Why does one blow the shofar during Musaf and not during the Yotzer (Shacharit)? Because by then Israel is already crowned with mitzvot.
“Since they are already wrapped in their tzitzit and have read the Shema and the Torah – now that the shofar is blown and the mitzvot testify in defence of Israel, Satan is immediately confounded and the accuser disappears’.”