Dormition or Assumption?
What actually happened when Mary died? In the East, the language of "falling asleep" seems to reflect Paul's when speaking of those who have already died and the story reflects Thomas' unique role as an apostle; in the West the promulgation into doctrine of the earlier tradition reflects the overall thinking about Mary and how God did things for her that now we all share, per Paul's language.
What happened? Who are we to say. No one wrote down her last moments. What we have is the Traditions of her death. In both East and West the truth remains: Mary died like all of us mortals die but then no body remained behind to undergo corruption.
Together we think of it as the "Firstfruits" Paul speaks of.
This feast celebrates something that we will all undergo. It does not single out Mary as different than us or more special. What Christ did, he did for us all, and if we look to the truth of Mary then we will see ourselves reflected in the greatness of God, who will raise us all up on the last day to share in what Christ and Mary have - the triumph over death!
But now Christ has been raised from the dead, the firstfruits of those who have fallen asleep. For since death came through a human being, the resurrection of the dead came also through a human being. For just as in Adam all die, so too in Christ shall all be brought to life, but each one in proper order: Christ the firstfruits; then, at his coming, those who belong to Christ; then comes the end, when he hands over the kingdom to his God and Father, when he has destroyed every sovereignty and every authority and power. For he must reign until he has put all his enemies under his feet. The last enemy to be destroyed is death....
-- 1 Corinthians 15:20-26