Exodus 12:21
Then Moses summoned all the elders of Israel and told them, "Go at once and select for yourselves a lamb for each family, and slaughter the Passover lamb.

Numbers 9:3
You are to observe it at the appointed time, at twilight on the fourteenth day of this month, in accordance with its statutes and ordinances."

Numbers 9:5
and they did so in the Wilderness of Sinai, at twilight on the fourteenth day of the first month. The Israelites did everything just as the LORD had commanded Moses.