Numbers 9:2-5

“The sons of Israel are to keep the Passover at its appointed time. On 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.” So Moses told the Israelites to observe the Passover. 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.