“On a seven day week, the first and the eighth are essentially the same day, with this one difference–the eighth day represents the re-creation or new creation. If you were an Old Covenant Israelite, reading the divine prescriptions concerning the observance of the Old Covenant ceremonies, you would be compelled to ask the question, “Why, if the Sabbath Day, from creation to the giving of the Law on Sinai, was the seventh day of the week, is there explicit reference to eighth day Sabbaths attached to the festivals?” 

In recent decades, the “eighth day” has been taken up by American pop-culture as something of a rhetorical literary device. When I was in high school there was a somewhat annoyingly catchy song about God making sweat tea on the eighth day. Then there was the Superbowl commercial about how God supposedly made farmers on the eighth day. While these attempts to employ the idea of the eighth day as an apparatus to show appreciation for the goodness of beloved objects, there is a divinely invested theological significance to the eighth day in Scripture–both with regard to the day on which the Israelite boys were to be circumcised (Genesis 17:12), as well as to the ceremonial Sabbaths in the Old Testament ceremonial law (e.g. the Feast of Tabernacles in Leviticus 23:36-39 and Numbers 29:35). The “eighth day” (in a seven-day week cycle) denotes new creation–one and eight representing creation and new creation.

In his disputation with Faustus, Augustine explained the change of signs from circumcision to baptism, and the change of the Sabbath day from the seventh to the eight, by suggesting that the “eighth day” in the Old Testament carried with it the idea of new creation and resurrection. He wrote:

[Christ] suffered voluntarily, and so could choose His own time for suffering and for resurrection, He brought it about that His body rested from all its works on Sabbath in the tomb, and that His resurrection on the third day, which we call the Lord’s day, the day after the Sabbath, and therefore the eighth, proved the circumcision of the eighth day to be also prophetical of Him.  For what does circumcision mean, but the eradication of the mortality which comes from our carnal generation?  So the apostle says:  “Putting off from Himself His flesh, He made a show of principalities and powers, triumphing over them in Himself.” The flesh here said to be put off is that mortality of flesh on account of which the body is properly called flesh.  The flesh is the mortality, for in the immortality of the resurrection there will be no flesh; as it is written, “Flesh and blood shall not inherit the kingdom of God.1

Notwithstanding his overly cautious caveat, John Calvin also indicated that he was attracted to this redemptive-historical idea regarding the “eighth day” for circumcision in the Old Covenant in his comments on Genesis 17:12:

Augustine also thinks that it had reference to the resurrection of Christ; whereby external circumcision was abolished and the truth of the figure was set forth. It is probable and consonant with reason, that the number seven designated the course of the present life. Therefore the eighth day might seem to be fixed upon by the Lord, to prefigure the beginning of a new life. But because such a reason is never given in Scripture, I dare affirm nothing. Wherefore, let it suffice to maintain what is certain and solid; namely, that God, in this symbol, has so represented the destruction of the old man, as yet to show that he restores men to life.2

This also fits in well with the idea of the eighth day Sabbaths at the appointed feasts and festivals in the ceremonial law. On a seven day week, the first and the eighth are essentially the same day, with this one difference–the eighth day represents the re-creation or new creation. If you were an Old Covenant Israelite, reading the divine prescriptions concerning the observance of the Old Covenant ceremonies, you would be compelled to ask the question, “Why, if the Sabbath Day, from creation to the giving of the Law on Sinai, was the seventh day of the week, is there explicit reference to eighth day Sabbaths attached to the festivals?” For instance, in Leviticus 23:36-39–at the institution of the Feast of Tabernacles, the LORD commanded Israel: