It's interesting to notice something here—and that is God's foresight. And, I might add here, also His plan. Remember that Abraham didn't have a child yet. But already God is planning a discipline that He is going to put the descendants of Abraham [through], in which they would be subject to others. But also planned was their release from that subjugation, the destruction of the Amorites under Joshua, and the inheritance of the Land. What God is showing is that the events of the exodus were part of a much larger plan, which God inaugurated through Abraham and then continued through Isaac, Jacob, Joseph and Moses. So let's go to chapter 17, where the real covenant was made with Abram.
Now let's jump, in the story here, to Exodus 19. We have gone from Exodus 12:40-41 with an emphasis on the 430 years, and underlining that is the language "that very same day." Then we jump back to Genesis 15 and pick up a promise which God sealed with a covenant that He would give to Abraham heirs from his own body to inherit the land. In chapter 17, we see the covenant given to Abraham. And now it is made an eternal covenant—an eternal inheritance of the land. And we find that sealed by circumcision—and, again, with language talking about the self-same day. In Exodus 19, the children of Israel are out of the land, and God is about to make the covenant with them—with the descendants of Abraham.
On that very day Abraham, Ishmael, and all the males of Abraham's household were circumcised, and thus they received the sign of the covenant. The covenant that was made at Mt. Sinai was essentially the same covenant as that entered into by God and Abraham, but expanded to include the entire nation (that is, all the descendants of Abraham). And added to it, then, were civil and ceremonial laws necessary for administering the covenant to the whole nation. That makes the 15th of Nisan (or, if you prefer, Abib) a very significant date.
