Numbers 9:3

9:3 In the fourteenth day of this month, at twilight, you are to observe it at its appointed time; you must keep it in accordance with all its statutes and all its customs.”

Numbers 9:5

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.