Numbers 9:1, 5, 11

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

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.

They may observe it on the fourteenth day of the second month at twilight; they are to eat it with bread made without yeast and with bitter herbs.