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.
What God did here was add circumcision as a requirement and as the sign that those bearing this mark had made the covenant at the same time with Abraham. (I believe that this term "selfsame day" appears something like fourteen times in the Bible.)