Today we celebrate the pledge of the resurrection of our human bodies: our belief that God took his Mother, Mary of Nazareth, body and soul to heaven at the end of her life. As she is now, so we someday hope to be.
There is always some angst over this feast day. We Romans of the West tend to think of it as "something we have to believe" because Pius XII said it was so in 1950. This is not the case, as Pius himself emphasizes repeatedly in his declaration (which is in itself a masterful and fascinating survey of the history of Church liturgy and theology).
The reasons we--and our brethren in the Eastern Churches--believe that Mary now lives in her resurrected body in heaven are based on the living traditions of the Church--our understanding of Scripture (particularly Revelation 12), the ancient liturgies and Church fathers, and the evidence of the earliest Christian churches in the Holy Land. Wikipedia actually has an amazing run-down of the what the feast is and is not for both the East and the West.
What the feast is not, for the West, is the commemoration of Mary's death. Pius XII left that question open--Roman Catholics are not bound to believe either that Mary died before her resurrection or that she was directly assumed into heaven without suffering death. The Eastern Churches, in general, hold--but not dogmatically, as is their custom--that she did physically die and then was raised to heaven on the third day after the manner of Christ himself. Pius XII chose not to define that dogmatically, for reasons unknown to this Philosopher Mom. The question was also left open by some Church Fathers, so maybe Pius just didn't see himself as setting in stone what the ancients left to heaven.
The point of today is not dogma. It is not wrangling. It is this: "As the most glorious Mother of Christ, our Savior and God and the giver of life and immortality, has been endowed with life by him, she has received an eternal incorruptibility of the body together with him who has raised her up from the tomb and has taken her up to himself in a way known only to him." ~St. John Damascene
"When this mortal thing hath put on immortality, then shall come to pass the saying that is written: Death is swallowed up in victory." ~St. Paul
As she is, may we also be! And to end: This icon (below) is also a favorite. Look! It's Jesus holding his mom's little tiny soul! How much do I just want to swaddle up and nestle in his arms? So sweet. So real.