Numbers 9:5-6

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.

Numbers 9:10

9:10 “Tell the Israelites, ‘If any of you or of your posterity become ceremonially defiled by touching a dead body, or are on a journey far away, then he may observe the Passover to the Lord.

Numbers 33:3

33:3 They departed from Rameses in the first month, on the fifteenth day of the first month; on the day after the Passover the Israelites went out defiantly in plain sight of all the Egyptians.