The odd Hebrew phrase “in the bone of the day” (translated as “the very same day”) occurs in Genesis 7 (Noah enters the ark), Genesis 17 (circumcision of Abram’s household), Exodus 12 (Passover), and Leviticus 23 (day of atonement).

Though the phrase is used a few other times in the OT, perhaps the repetition of the phrase joins these events together.  In the flood, the world is “circumcised” and we have a foreshadowing of the Passover and the ultimate atonement.

What makes these “bone” days?  Literally, the “bone of the day” has been taken as a reference to noonday - the “strength” of the day.  That may be, but why describe strength as “bone”?  Bones are  structuring organs for the human body.  The bone of days are days that provide a skeletal structure to time.