Explanation of The Assumption of the Blessed Virgin Mary provided by LPi, Liturgical Publications, inc:
This feast is a very old solemnity in the church. It has been celebrated throughout the church from the sixth century on. Originally celebrated in the East, the ancient title is the Dormition of Mary, or the Falling Asleep of Mary. The earliest writing we find of this feast is from the fourth century and it is written in the voice of St. John, whom Jesus entrusted Mary to the care of as he hung upon the cross.
The teaching is that Mary was assumed into heaven after her "falling asleep in the Lord" before her body suffered any decay. This doctrine of the church--even though the church has been celebrating this belief for centuries--was not declared a dogma until November 1, 1950 by Pope Pius XII. While the Eastern Orthodox call this event the "dormition," the Roman Catholic Church calls it the Assumption.
Roman Catholicism is silent on the details of the Assumption. Therefore, Roman Catholics legitimately can believe that Mary did not die before the Assumption. The holy day has always been celebrated in the church on August 15.
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"My soul proclaims the greatness of the Lord; my spirit rejoices in God my Savior for he has looked with favor on his lowly servant. From this day all generations will call me blessed." - Lk 1:46-48