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.
We will keep establishing this pattern of the “bone day,” and the next one is in Genesis 17:
This was another highly significant day in God’s outworking, and God underscores it by mentioning the “very same day” twice. This one is harder to nail down, but we can begin with the fact that Genesis 17 does not stand on its own. There was already a history. There were events and divine pronouncements that came before and resulted in this “bone day” and proved God’s faithfulness. Perhaps it was the covenant in Genesis 15.
Another possibility—and, I believe, a better one—goes back even farther, to the first time God appeared to Abram when he got to Canaan. On that occasion, God promised, “To your descendants I will give this land” (Genesis 12:7). It was a promise of both inheritance as well as descendants. There is an echo of that promise on this esem day in verse 8 here, where God says,
So, when Abram got to the land, God said, “To your descendants I will give this land,” and on this esem day, God confirms it. His words change from “I will give” to “I give” as He faithfully fulfilled His promise from decades before.
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.)
