Numbers 9:1, 5, 11

A year after Israel’s departure from Egypt, the LORD spoke to Moses in the wilderness of Sinai. In the first month of that year he said

in the wilderness of Sinai as twilight fell on the fourteenth day of the month. And they celebrated the festival there, just as the LORD had commanded Moses.

They must offer the Passover sacrifice one month later, at twilight on the fourteenth day of the second month. They must eat the Passover lamb at that time with bitter salad greens and bread made without yeast.