Numbers 9:2

The LORD spoke to Moses in the wilderness of Sinai, in the first month of the second year after they had come out of the land of Egypt, saying,

On the fourteenth day of this month, at evening, you shall keep it in its appointed season. You shall keep it according to all its statutes and according to all its ordinances.”

Moses told the children of Israel that they should keep the Passover.

They kept the Passover in the first month, on the fourteenth day of the month at evening, in the wilderness of Sinai. According to all that the LORD commanded Moses, so the children of Israel did.