This is a reference to the future time when the Christians receive their resurrected bodies. Our salvation includes the future glorification of our fleshly abodes.
Phil. 3:20-21, “For our citizenship is in heaven, from which also we eagerly wait for a Savior, the Lord Jesus Christ; 21 who will transform the body of our humble state into conformity with the body of His glory, by the exertion of the power that He has even to subject all things to Himself.”
Jesus is called the firstborn of creation. He is also the first fruits of the resurrections of whom we follow.
- Rom. 8:29, “For whom He foreknew, He also predestined to become conformed to the image of His Son, that He might be the first-born among many brethren.”
- Col. 1:15, “And He is the image of the invisible God, the first-born of all creation.”
- 1 Cor. 15:20-23, “But now Christ has been raised from the dead, the first fruits of those who are asleep. 21 For since by a man came death, by a man also came the resurrection of the dead. 22 For as in Adam all die, so also in Christ all shall be made alive. 23 But each in his own order: Christ the first fruits, after that those who are Christ’s at His coming.”
The future resurrection of our bodies is also referred to as receiving our glorified bodies. This event is so certain (because of the sacrifice of Christ) that our future glorification is spoken of in the past tense:
- Rom. 8:30, “and whom He predestined, these He also called; and whom He called, these He also justified; and whom He justified, these He also glorified.”
So, the Day of Redemption is that future day when we will all be resurrected, changed, and receive our resurrected bodies.