Midrash sa Levitico 23:35: Sifra

1) I might think that (the above) are permitted (to be brought on the festival, but are not mandatory); it is, therefore, written (of the same) (Bamidbar 29:39) "These you shall offer to the L–rd on your festivals." If to permit, they have already been permitted (according to our assumption). If so, why is the latter stated? To make them mandatory — that all of them must be brought on the festival.

2) I might think (that he could bring them) on any festival that he desired. It is, therefore, written (Devarim 12:5-6) "And you shall come there (to the Temple) and you shall bring there" (the same). If to permit, it had already been permitted. If to make mandatory, it has already been made mandatory. If so, why is it written? (To make it mandatory for him to bring them) on the first festival (of the three festivals) that arrives (after they were due).

3) I might think that if one festival passed and he did not bring them, he transgresses (Devarim 23:22) "You shall not delay (to pay it.") It is, therefore, written (Bamidbar 29:29) "These shall you offer to the L–rd on your festivals." He does not transgress "You shall not delay" until all the (three) festivals of the year have passed. R. Shimon says: Three consecutive festivals, with the festival of matzoth first.