The feast of the Dormition celebrates the falling asleep of the Theotokos (Mary, Mother of God). Celebrated on different dates since the fourth century, it was formally established on August 15 by Emperor Maurice (ruled approx 582-602 A.D.). The Feast of Dormition was introduced in Rome in the 7th century and eventually spread to the entire West, where it became known as the Feast of the Assumption of the Blessed Virgin Mary into Heaven.
At this feast flowers are blessed; according to tradition when the tomb of the Blessed Virgin was opened on the third day, it was found to be empty except for sweetly fragrant flowers.
On November 1, 1950, Pope Pius XII proclaimed 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.”
The following is excerpted from Julian J. Katrij, A Byzantine Rite Liturgical Year:
Three days before the death of Mary, the Archangel Gabriel appeared and announced to her from her Son, Jesus Christ, the time of her departure into eternity. On the day of her death, the Apostles gathered in Jerusalem, although they had been scattered in various countries of the then known world. St. Thomas was the only Apostle absent. The Mother of God expressed her wish to be buried in Gethsemane near her parents and her spouse, Joseph. Christ, in person, came accompanied by Angels and Saints to escort His Most Holy Mother to heaven. The Apostles, while singing holy hymns, carried the body of Mary to the grave where they remained keeping vigil for three days. On the third day, St. Thomas came from afar and desired very much to view for the last time the Most Pure Mother of God. When the grave was opened, her body was not there, only the funeral clothes in which the body had been wrapped. The Apostles then realized that Mary had been taken up body and soul into heaven.