Charles Ellicott Commentary Exodus 12:1

"And Jehovah spake unto Moses and Aaron in the land of Egypt, saying," — Exodus 12:1 (ASV)

In the land of Egypt.—This section (Exodus 12:1–28) appears to have been written independently of the previous narrative—probably earlier, and as a part of the Law rather than of the history. It combines instructions on the subject of the Passover which must have been given at different times (Exodus 12:12; Exodus 12:17), some before the tenth of Abib, some on the day preceding the departure from Egypt, and some on the day following.

Up to Exodus 12:20, the section is wholly legal and would suit Leviticus as well as Exodus. From Exodus 12:20 onward, it has a more historical character, since it relates the action taken by Moses.