By the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.
-Pope Pius XII in Munificentissimus Deus on November 1, 1950
August 15th is the Feast of The Assumption of The Virgin Mary, although the feast is ancient, the doctrine was only dogmatically defined in 1950 by Pope Pius XII. Does this mean that Pope Pius XII just invented this belief? Not at all!
When a doctrine is dogmatically defined it doesn't mean it was just invented. The Church never "invents" doctrine, but sometimes a doctrine is accepted and believed by the faithful, but has never been officially and infallibly defines by the Church. This is what took place in 1950, and many times throughout Church history, when the Church made official and infallible, what it had already taught for centuries.
This feast has been observed since at least the 6th century, and probably much earlier than that. The belief in the Virgin Mary's assumption has been a part of the church since the days of the Apostles. Although this doctrine is not supported by direct scriptural the evidence, it is nonetheless supported by implied scriptural proofs.
This Marian dogma, like the other three of the four Marian dogmas, is often misunderstood by Catholics and misconstrued by non Catholics. Lets first look at what it is not.
What the assumption is not
Assumption is not the same as Ascension. Jesus ascended into heaven, to ascend is to rise up by your own power, so Jesus raised himself up to heaven by His own power. Assumption means to be pulled up by another, this means that Mary was assumed by the power of God.
To say that Mary ascended into heaven is not just grammatically incorrect, it is also theologically wrong. If Mary ascended then she had the power to ascend to heaven, only God has that power, and Mary is not God. So Mary was assumed into heaven, by the power of God, in a way similar to Enoch and Elijah.
Persons being assumed into heaven is Biblically based
To those that argue Mary's assumption is a far fetched proposal, just look at Enoch and Elijah. Both of the Old Testament characters were bodily assumed into heaven. Although the word "assumption" or "assumed" isn't used, the same principle is in effect.
Enoch walked with God; and he was not, for God took him.
This verse doesn't end with the "and he died", that is include after every other person in the genealogy of Genesis 5. While this doesn't exactly prove that Enoch was assumed into heaven, the fact that it doesn't mention his death, but mentions the death of every other person in the genealogy, does lend this theory credence.
The next Old Testament instance of assumption is much more clear.
And as they still went on and talked, behold, a chariot of fire and horses of fire separated the two of them. And Eli'jah went up by a whirlwind into heaven.
2 Kings 2:11
Not to say that there even needs to be proof (of the Church teaches it then I believe it), but this verse seems to prove the concept of assumption is a viable one.
The dormation of the Virgin Mary
Did Mary die an earthly death before her assumption into heaven? Or was she assumed while still alive? I don't know, and the church doesn't officially hold either position.
The Eastern Orthodox churches hold to a doctrine called the Dormation of the Virgin Mary, this docrtine proposes that she fell into a deep coma like sleep and then was assumed three days later. The Catholic Church neither condemns or endorses this belief, in his dogmatic defining of the doctrine of the assumption, Pope Pius XII intentionally left this issue open.
I believe that the historical evidence points toward Mary having died and then being assumed into heaven, this would also be more theologically consistent that she'd suffer an earthly death just like her Son. But faithful Catholics are free to believe in any of the three available options; that she died, that she fell into a dormation, or that she was assumed while still alive.
The tomb of the Virgin Mary
If you've ever visited Jerusalem you probably would've seen the tomb of the Virgin Mary. You would notice that it is an empty tomb, with some of the earliest church traditions saying that the Blessed Mother died while the Apostle Thomas was away, and when he returned to Jerusalem a few days later they opened the tomb for him to pay his respects, and it was empty.
There are other apocryphal stories and traditions pertaining to the assumption of Mary, but the one thing that is conspicuously missing is a claim on her body. You will not find any church, museum or archeological site that claims to contain the bones of the Virgin Mary. No early local church claimed to have them, no church father wrote about where they were for the purpose of veneration. Considering that the Virgin Mary has been held in high esteem since the founding of the church, and that from those earliest days people have prayed over the bones of the saints, it is quite telling that there is never a claim that someplace held her body.
Why the Assumption is important
CCC 966: "Finally the Immaculate Virgin, preserved free from all stain of original sin, when the course of her earthly life was finished, was taken up body and soul into heavenly glory, and exalted by the Lord as Queen over all things, so that she might be the more fully conformed to her Son, the Lord of lords and conqueror of sin and death." The Assumption of the Blessed Virgin is a singular participation in her Son's Resurrection and an anticipation of the resurrection of other Christians.
God chose to assume the Virgin Mary into heaven, and God doesn't do anything without reason. The Church teaches us that the Assumption is "an anticipation of the resurrection of other Christians," what this means is that just as we can look to the life of Mary as an example of faith and holiness, so to can we look at her death and assumption as an example of what we can expect.
Mary, being the most perfect of all God's creations, is the ultimate example of Christian virtue, and she is also the ultimate foreshadowing of things to come. We can all look forward to the resurrection of the dead, and the life of the world to come, by looking at Christ's Blessed Mother.
Our Lady of The Assumption, pray for us.