It must be remembered that it is not the assumption that the Holy Orthodox Church observes on the 15th of August, but the Feast of the Dormition (Keemeesis or Koimisis) or “falling asleep” of the holy Virgin. This most sacred feast marks the falling asleep of the Mother of God, which was followed by the translation of her sacred body three days later into heaven. This feast, therefore, marks her soul being commended into her Son’s hands and the short sojourn of her body in the tomb. Death is not the annihilation of our existence, but a passage from earth to heaven.

It was Saint Juvenal (5th C.), Patriarch of Jerusalem, who related to Saint Pulcheria the earliest traditions concerning the translation of the relics of the Theotokos. At the end of the sixth century, Emperor Maurice (582-602), dated the Virgin Theotokos’ august Dormition as the 15th of August. He ordered that the feast be celebrated throughout the universe. Unlike the resurrection of Christ, the mysterious character of her death, burial, resurrection, and ascension were not the subject of apostolic teachings, or at least there are no extant epistles; yet it has been revealed to the inner consciousness of the Church. Inaccessible to the view of those outside the Church, the glory of the Theotokos’ Dormition can be contemplated only in the inner light of Tradition. The glorification of the Virgin-Mother is a result of the voluntary condescension of the Son Who is incarnate of her and made “Son of Man,” capable of dying. The Mother of God is now established beyond the general resurrection and the last judgment, having passed from death to life, from time to eternity, from a terrestrial condition to celestial beatitude. Hence, the feast of August 15th is a second mysterious Pascha, since the Church celebrates, before the end of time, the secret firstfruits of its eschatological consummation.

Source: The Great Synaxaristes of the Orthodox Church, August. Holy Apostles Convent.