The Lord spoke to Moses in the Tent of Meeting while they were in the Sinai desert. This was on the first day of the second month of the second year after the Israelites had left Egypt. He told him,
You are to observe it in the evening after sunset on the fourteenth day of the second month. You are to eat the lamb with the bread made without yeast, and the bitter herbs.
So they kept the Passover in the Sinai desert, beginning in the evening after sunset on the fourteenth day of the first month. The Israelites followed all the instructions the Lord had given to Moses.