The Roman Catholic doctrine of the assumption of Mary teaches that she was assumed body and soul into heaven either without dying or shortly after death. This extraordinary claim was only officially declared to be a dogma of Roman Catholic faith in 1950, though it had been believed by many for hundreds of years. To dispute this doctrine, according to Rome’s teaching, would result in the loss of salvation. The official teaching of the Assumption comes from the decree Munificentissimus Deus by pope Pius XII:
All these proofs and considerations of the holy Fathers and the theologians are based upon the Sacred Writings as their ultimate foundation. These set the loving Mother of God as it were before our very eyes as most intimately joined to her divine Son and as always sharing His lot. Consequently it seems impossible to think of her, the one who conceived Christ, brought Him forth, nursed Him with her milk, held Him in her arms, and clasped Him to her breast, as being apart from Him in body, even though not in soul, after this earthly life. Since our Redeemer is the Son of Mary, He could not do otherwise, as the perfect observer of God’s law, than to honour, not only His eternal Father, but also His most beloved Mother. And, since it was within His power to grant her this great honour, to preserve her from the corruption of the tomb, we must believe that He really acted in this way.
Hence the revered Mother of God, from all eternity joined in a hidden way with Jesus Christ in one and the same decree of predestination, immaculate in her conception, a most perfect virgin in her divine motherhood, the noble associate of the divine Redeemer who has won a complete triumph over sin and its consequences, finally obtained, as the supreme culmination of her privileges, that she should be preserved free from the corruption of the tomb and that, like her own Son, having overcome death, she might be taken up body and soul to the glory of heaven where, as Queen, she sits in splendor at the right hand of her Son, the immortal King of the Ages.
For which reason, after we have poured forth prayers of supplication again and again to God, and have invoked the light of the Spirit of Truth, for the glory of Almighty God Who has lavished His special affection upon the Virgin Mary, for the honour of her Son, the immortal King of the Ages and the Victor over sin and death, for the increase of the glory of that same august Mother, and for the joy and exultation of the entire Church; 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.
Hence, if anyone, which God forbid, should dare wilfully to deny or call into doubt that which we have defined, let him know that he has fallen away completely from the divine and Catholic faith...It is forbidden to any man to change this, Our declaration, pronouncement, and definition or, by rash attempt, to oppose and counter it. If any man should presume to make such an attempt, let him know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul (Munificentissimus Deus, Selected Documenst of Pope Pius XII (Washington: National Catholic Welfare Conference), 38, 40, 44-45, 47).
This is truly an amazing dogma, yet there is no Scriptural proof for it, and even the Roman Catholic writer Eamon Duffy concedes that, ‘there is, clearly, no historical evidence whatever for it ...’ (Eamon Duffy, What Catholics Believe About Mary (London: Catholic Truth Society, 1989), p. 17). For centuries in the early Church there is complete silence regarding Mary’s end. The first mention of it is by Epiphanius in 377 A.D. and he specifically states that no one knows what actually happened to Mary. He lived near Palestine and if there were, in fact, a tradition in the Church generally believed and taught he would have affirmed it. But he clearly states that ‘her end no one knows.’ These are his words:
But if some think us mistaken, let them search the Scriptures. They will not find Mary’s death; they will not find whether she died or did not die; they will not find whether she was buried or was not buried ... Scripture is absolutely silent [on the end of Mary] ... For my own part, I do not dare to speak, but I keep my own thoughts and I practice silence ... The fact is, Scripture has outstripped the human mind and left [this matter] uncertain ... Did she die, we do not know ... Either the holy Virgin died and was buried ... Or she was killed ... Or she remained alive, since nothing is impossible with God and He can do whatever He desires; for her end no-one knows.’ (Epiphanius, Panarion, Haer. 78.10-11, 23. Cited by juniper Carol, O.F.M. ed., Mariology, Vol. II (Milwaukee: Bruce, 1957), pp. 139-40).
http://www.christiantruth.com/assumption.html
This holy day of obligation follows the tradition of recognizing Mary's death and resurrection. According to legend, when Mary was 73 years old and on her death bed, all the apostles except St. Thomas gathered with Mary as she passed on. When Thomas arrived he asked to see her body but the body was not there for it too had been resurrected.
While the apostles watched, a cloud descended over Mary and transported her body to Jerusalem. Meanwhile, the Angel Gabriel took her spirit to Heaven. Both her body and spirit were reunited once again in her tomb in the Valley of Jehoshaphat.
This holiday has been celebrated for many years by Christians. Some believe the date of August 15 was set to coincide with popular pagan harvest festivals of the time. It wasn't for many centuries of popular request, before Pope Pius XII declared Mary's assumption to be dogma or a fundamental belief of Catholics. This declaration occurred on November 1, 1950.
http://www.holidayorigins.com/html/assumption_of_mary.html