Numbers 9:4-6
9:4 So Moses instructed the Israelites to observe the Passover.
9:5 And they observed the Passover on the fourteenth day of the first month at twilight in the wilderness of Sinai; in accordance with all that the Lord had commanded Moses, so the Israelites did.
9:6 It happened that some men who were ceremonially defiled by the dead body of a man could not keep the Passover on that day, so they came before Moses and before Aaron on that day.