I might think that the only things sacrificed on a festival are the sacrifices of the festival alone. Whence are derived for inclusion communal offerings, and individual offerings dedicated before the festival and on the festival itself? From (Ibid. 38) "aside from your gifts and aside from all your vows and aside from all your gift-offerings that you give to the L-rd" — to include birds and meal-offerings, all of which are sacrificed on the festival.
I might think that this is optional; it is, therefore, written (Bamidbar 29:39) "These shall you offer to the L-rd on your festivals." If to permit them, they have already been permitted. If so, why is it written "These shall you offer to the L-rd on your festivals"? To make them mandatory, that all are to be sacrificed on the festival.