Numbers 9:2-3

9:2 Let the children of Israel also keep the passover at his appointed season. 9:3 In the fourteenth day of this month, at even, ye shall keep it in his appointed season: according to all the rites of it, and according to all the ceremonies thereof, shall ye keep it.

Numbers 9:7

9:7 And those men said unto him, We [are] defiled by the dead body of a man: wherefore are we kept back, that we may not offer an offering of the LORD in his appointed season among the children of Israel?

Numbers 9:13

9:13 But the man that [is] clean, and is not in a journey, and forbeareth to keep the passover, even the same soul shall be cut off from among his people: because he brought not the offering of the LORD in his appointed season, that man shall bear his sin.