In Christianity the concept of the 8th day of creation is the day Christ rose from the dead, as such the number 8 has figured in Christian art and architecture to symbolize renewal--the most common expression of this is the use of octagonal shaped baptismal fonts.
The earliest mention of an 8th day comes from the Epistle of Barnabas, one of the ancient books of the antilegomena (books disputed in the early Church which were sometimes accepted as canonical and sometimes not accepted as canonical),
"Wherefore also we keep the eighth day for rejoicing, in the which also Jesus rose from the dead, and having been manifested ascended into the heavens." - Barnabas 15:9
St. Justin references it a few times in his Dialogue with Trypho, notably here:
"You know, then, sirs, that God has said in Isaiah to Jerusalem: 'I saved you in the deluge of Noah.' By this which God said was meant that the mystery of saved men appeared in the deluge. For righteous Noah, along with the other mortals at the deluge, i.e., with his own wife, his three sons and their wives, being eight in number, were a symbol of the eighth day, wherein Christ appeared when He rose from the dead, for ever the first in power. For Christ, being the first-born of every creature, became again the chief of another race regenerated by Himself through water, and faith, and wood, containing the mystery of the cross; even as Noah was saved by wood when he rode over the waters with his household." - Dialogue with Trypho, ch. 138
Justin earlier had drawn significance from circumcision occurring on the eighth day as a point of symbolic importance; and the significance of baptism/regeneration as, in Christian thought, spiritual circumcision in Christ.
My point is really only this: In Christianity there is already a great deal of theological import put on the concept of an 8th day of creation that is found exclusively in our confession of Jesus' resurrection and the renewal, restoration, and redemption of all creation through Him. From a Christian perspective every time we gather, on the first day of the week, to receive Word and Sacrament, we are celebrating the 8th day of creation. The first day of the week is, for us, spiritually the 8th day, or the Lord's Day, the day of the resurrection, the day of redemption; pointing back to Christ's own resurrection, our present salvation, and looking forward to the day when God makes all things new at the Eschaton.
-CryptoLutheran