Midrasch zu Bamidbar 9:4

(Bamidbar 9:4) "And Moses spoke to the children of Israel to offer the Pesach": Why is this stated? It is already written (Vayikra 23:44) "And Moses declared the festivals of the L-rd to the children of Israel." What, then, is the intent of "And Moses spoke to the children of Israel to offer the Pesach"? Keep the Pesach in its appointed time (viz. Devarim 16:1), so that all of the festivals fall out in their proper season. Variantly: We are hereby taught that he heard the sections of the festivals at Sinai, related them to Israel, and repeated them before their performance. Variantly: He told them the halachoth of Pesach before Pesach, the halachoth of Shavuoth before Shavuoth, and the halachoth of Succoth before Succoth — whence they said: Moses instituted for Israel that they ask and expound re the festival (preceding the festival).