Numbers 9:2-5

2“The sons of Israel are to keep the Passover at its appointed time.

3On the fourteenth day of this month at twilight, you shall keep it at its appointed time; according to all its statutes and ordinances you shall keep it.”

4So Moses told the Israelites to observe the Passover.

5They 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.