"From eternity Christ is born, he is always being born."
Quicquid ab aeterno nascitur, semper nascitur WA 39/II, 293.
Luther seems at some points to think of the nativity in similar terms to the crucifixion, which is likely of a piece with the broader implications of Luther's being a theologian of the cross keeping connected what theology sometimes separates- incarnation, crucifixion, resurrection.
Instead, just as the cross was, yet is for us now, and continues in the future(see Paul in Colossians), so to Christ was born, is being born, and will be born. This is not first of all a liturgical point, but a matter of faith, and the life of faith.
"Everything is full of Christ through and through, even according to his humanity" (LW 37, 151-372).
"Thus it is rightly and truly said: God is born, was nursed or suckled, lay in the crib, felt cold, walked, stood, fell, wandered, ate, drank, suffered, died, etc." (Tischreden, VI:68, 18-40).
I remember reading an essay once with the title, "The Incarnation Never Saved Anyone." The thesis troubled me now and still does. The point was to try and lift up the cross and resurrection as signs of forgiveness and salvation, with the incarnation as more of a necessary precursor. But if we simply remember that it is the same body, the same person, born there of Mary who is later crucified on the cross, and it is the same body that God raises from the dead, then we cannot say so simplistically that the Incarnation never saved anyone.
Remember was Gregory Nanzianzen wrote: "For that which He has not assumed He has not healed; but that which is united to His Godhead is also saved" (Nicene and Post-Nicene Fathers, Hendrickson, 1995, 7:440). Christ at birth, or even better, at the conception of Mary and the Holy Spirit, unites God and humanity, and this is already at this point good news, that God has put on that which has gone astray in order to save it.