Because the celebration of the Eucharist introduces us to the "final day" (
eschata), the Lord's Day is also known as the "eighth day," i.e. the day which will have "no need of sun or moon to shine upon it, for the glory of God is its light and its lamp is the Lamb" (Rev. 21: 23). The "eighth day" is a term which indicates the final age, when the new creation, already begun by the resurrection of Christ, will be fulfilled and completed; when the new world will be ushered in by the general resurrection.  (Goarch.org)
Fr. Joseph Gleeson writes in his blog 
The Eighth Day
On Holy Saturday, His body rests in the tomb as He keeps the final Sabbath in the Old Covenant. Then, on the 8th day. . .
- On the 8th day death is defeated.
 
- On the 8th day, the gates of Hell are shattered.
 
- On the 8th day, the graves release their captive.
 
- On the 8th day, the Son of God rises from the dead holding the keys of death and hell in one hand and the broken teeth of the devil in the other.
 
- On the 8th day, the old creation begins passing away, and the new creation begins breaking into our present world, for when the Apostles look upon the Risen Christ, they are looking at a human body which has already crossed over from death into life.
 
- They are looking at a human body that will outlive this present universe. When this world burns, and we meet the end  of the world as we know it, our Risen Savior will still be there, alive and well on the other side.