The first thing that comes to mind, which prehaps merits some discussion, is the question of what we call today's feast. Does the designation one gives to this holy day tell us about his thelogy, or of what he thinks of Mary? Not necessarily all that much, though I would say that some designations are less advisable than others. The modern day Missouri Synod Lutheran has a few choices to make.
He could stick with his church's official literature, which prefers to say that this is the feast of Saint Mary, Mother of our Lord, a term which is not heretical, so it passes the all important Doctrinal Review process of the Synod, but which does have a couple of problems. One is that it seems to be a generic Marian feast, not specific to any event or mystery in her life, as if this were the only day for Mary, which is simply not the case, not even in modern Lutheran service books, in which the Annunciation, Visitation, and Purification are all kept. In fact, this feast specifically is meant to celebrate the outcome of Mary's earthly life, the fact that she fell asleep in Christ, and is now with her Lord. The second problem with the Missouri Synod's preferred designation, Mary, Mother of our Lord, is that, while true in itself, it is most noteworthy that Nestorius also would have been quite pleased with it, for it gets around having to face the question of Mary being the Mother of God. Since Mary's Son is God Himself made flesh, the creator of all things, the Church in her wisdom decided that, indeed, it is right and proper to say that Mary is the Mother of God. Why is the Missouri Synod unwilling to call Mary the Mother of God? Surely the Synod is not dogmatically opposed to it, her defenders will tell me. I fail to see the defense, however, for erecting a wall between dogma and practice.
The LCMS Lutheran could call it the feast of Mary's Assumption. Most Lutherans, even of those dissatisfied with the Missouri Synod's practice, shy away from this, for a variety of reasons. Not all have all the same reasons. You would really need to ask each Lutheran who dislikes using "Assumption" just why it is that he feels that way. Some say, for example, that it is Roman Catholic. Lutherans are not Catholic. Therefore it is unLutheran terminology. This logic has many problems, but for now let us just point out that Mary's Assumption is not something that has ever been condemned by the Lutheran Church. In fact, there is a history of this feast being kept as the Assumption in the Reformation churches. As Professor Joseph Herl shows in his book, Worship Wars in Early Lutheranism, the Assumption is listed in the church orders in Weissenburg 1528, Dessau 1532, Nordlingen 1538, Brandenburg 1540, Palatinate-Neuburg 1543, Schwabisch Hall 1543, Brandenburg-Ansbach-Kulmbach 1548, Hohenlohe 1553, & Nuremberg 1543. To dogmatize the Assumption, as Rome did in 1950, is not the sort of thing the Church of the Augsburg Confession would do, yet it is quite another thing to claim that the Assumption itself (the event, not the dogma) is impossible and unLutheran.
There is another option, namely, to see this as the feast of Mary's Dormition. Some see this as too Byzantine, too Orthodox. Here we must clarify a few things. One is that there is no conflict or contradiction between the Assumption and the Dormition, as if one necessarily cancels the other out. Many of those who, even in the ancient church, believed and celebrated and preached the Dormition also believed that Mary was taken bodily to heaven. Likewise many who believe in Mary's assumption also believe that she did in fact die. Even the Roman Church's official definition of the dogma of the Assumption allows for Mary's death at the end of her earthly life, contrary to what I've heard some claim about that dogma. Many do prefer to simply celebrate this, though, as Mary's Dormition, and to be content that she is now in heaven.
In Mary's death we see the perfect image of the Christian who falls asleep in Christ, who finds perfect rest in him. Mary's death is the Happy Death (bona mors), for which we pray.
Here is what Saint John of Damascus preached on this feast over a millenium ago:
"Today the sacred and living ark of the living God, she who conceived the Creator in her womb, comes to rest in the temple of the Lord which was not made by men's hands...Today the Eden of the New Adam receives the living paradise in which our condemnation was dissolved, in which the tree of life was planted, in which our nakedness was clothed."