As far back as the 6th century, there has been a clear belief in the Church, especially in the East, that the Blessed Virgin Mary was immediately assumed into heaven at the end of her life on earth. In 1950, Pope Pius XII at long last, after a broad consultation of bishops, theologians and laity, finalized this by declaring "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 to heavenly glory."
This feast is celebrated under various names – in the East still the more favored title is that of Mary's Dormition, her "falling asleep," emphasizing that we do not know if at the end of her life on earth the Blessed Virgin actually died or not. But it is universally celebrated.
The Bible gives witness to Mary’s Assumption into heaven indirectly: Revelation 12 speaks of a woman who is caught up in the battle between good and evil. This woman traditionally has been understood to be Mary. Furthermore, in 1 Corinthians 15:20, St. Paul taught that Christ’s resurrection is the "first fruits of those who have fallen asleep," and Mary chief among all human beings deserves the reward that we all hope to attain.
It is not surprising that the Holy Spirit has led the Church to believe in Mary's already full share in His glorification. So close was Hshe to Jesus on earth, she must be with Him body and soul in heaven.