And yet, this is what Christians from the beginning have believed.
That none of the canonical books of the Bible explicitly use this phrase, "the eighth day" does not negate the meaning and significance of what it refers to. In the same way that we may not find "the Holy Bible" or "the Canon of Scripture" or "the Trinity" et al does not negate the truthfulness of what is being spoken of.
Taking what is said in Scripture, in the whole, this is what the earliest Christians believed and understood. And we know that, because that's what they themselves said.
"Furthermore He says to them, 'Your new moons and sabbaths I cannot away with.' Do you see what He means? The present sabbaths are not acceptable to Me, but that which I have made, in which I will give rest to all things and make the beginning of an eighth day, that is, the beginning of another world. Wherefore we also celebrate with gladness the eighth day in which Jesus also rose from the dead, and was made manifest, and ascended into heaven." - Epistle of Barnabas 15:8-9, c. 70-120 AD
"Justin: Now, sirs, it is possible for us to show how the eighth day possessed a certain mysterious import, which the seventh day did not possess, and which was promulgated by God through these rites [i.e. circumcision, ritual cleansing, et al]. But lest I appear now to diverge to other subjects, understand what I say: the blood of that circumcision is obsolete, and we trust in the blood of salvation; there is now another covenant, and another law has gone forth from Zion. Jesus Christ circumcises all who will--as was declared above--with kives of stone; that they may be a righteous nation, a people keeping faith, holding to the truth, and maintaining peace. Come then with me, all who fear God, who wish to see the good of Jerusalem. Come let us go to the light of the Lord, for He has liberated His people, the house of Jacob. Come, all nations; let us gather ourselves at Jerusalem, no longer plagued by war for the sins of her people. 'For I was manifest to them that sought Me not; I was found of them that asked not for Me,' He exclaims by Isaiah: 'I said, Behold Me, unto nations which were not called by My name. I have spread out My hands all the day unto a disobedient and gainsaying people, which walked in a way that was not good, but after their own sins. It is a people that provoked Me to My face.'" - St. Justin Martyr, Dialogue with Trypho, ch. 24, c. 150 AD
"For in respect of the observance of the eighth day in the Jewish circumcision of the flesh, a sacrament was given beforehand in shadow and in usage; but when Christ came, it was fulfilled in truth. For because the eighth day, that is, the first day after the Sabbath, was to be that on which the Lord should rise again, and should quicken us, and give us circumcision of the spirit, the eighth day, that is, the day after the Sabbath, and the Lord's day, went before in the figure; which figure ceased when by and by the truth came, and spiritual circumcision was given to us." - St. Cyprian of Carthage, Epistle 58, ch. 4, c. ~230-250 AD
"Before the arrival of the eighth day of the Lord Jesus Christ the whole world was pure and uncircumcised. But when the eighth day of the resurrection came, immediately we were cleansed, buried, and raised by the circumcision of Christ." - Origen of Alexandria, On the Psalms, c. 250 AD
As you can see, they did not pull the idea of the "eighth day" from thin air. It is built upon the Scriptures; for they understood that God had used the eighth day in the past to signify important things. This is why God instructed the Israelites to circumcise on the eighth day, this is why they were to cleanse themselves on the eighth day. And it is in this type of the eighth day, which the Apostles themselves draw from, as St. Paul does to speak of Baptism as our true circumcision "made without hands", and our purification from sins in Christ. That it is the eighth day in which newness and beginnings are found. It is the day of new identity and new life--having been made purified and a new creation in the death and resurrection of Christ.
This is why it is the eighth day. It is biblical.
-CryptoLutheran