John Calvin Commentary Deuteronomy 16:3

"Thou shalt eat no leavened bread with it; seven days shalt thou eat unleavened bread therewith, even the bread of affliction; for thou camest forth out of the land of Egypt in haste: that thou mayest remember the day when thou camest forth out of the land of Egypt all the days of thy life." — Deuteronomy 16:3 (ASV)

Thou shalt eat no leavened bread with it. Because by this sign they were reminded of their having escaped in haste, as if from the very flames, Moses therefore so often enforces the prohibition of leaven. And here this reason for it is given, namely, that their minds should be recalled to the affliction from which they were rescued; for they must necessarily have been involved in the greatest difficulties, when there was no time even for baking bread. Unleavened bread is therefore called “the bread of affliction,” so that the manner of their deliverance may further enhance God’s grace.

He repeats what we have seen before: that none of the flesh of the Lamb should be reserved for the following day. In the former passage from the Book of Exodus, because Moses speaks generally, the command may at first sight be referred to the perpetual sacrifice; however, the latter passage removes all obscurity by the express mention of the Passover.

We need not wonder that in one place the word “fat” is used for the whole carcass, or any part of the lamb, either by synecdoche, or so that God might commend the superior sanctity of the fat, which they were not permitted to eat, and which was burned in all sacrifices.