Numbers 9:5 (KJV)

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.

And Moses spake unto the children of Israel, that they should keep the passover.

And they kept the passover on the fourteenth day of the first month at even in the wilderness of Sinai: according to all that the Lord commanded Moses, so did the children of Israel.

And there were certain men, who were defiled by the dead body of a man, that they could not keep the passover on that day: and they came before Moses and before Aaron on that day:

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?