The Feast of the Dormition of Our Most Holy Lady, the Theotokos and Ever-Virgin Mary is celebrated on August 15 each year. The Feast commemorates the repose (dormition and in the Greek kimisis) or "falling-asleep" of the Mother of Jesus Christ, our Lord. The Feast also commemorates the translation or assumption into heaven of the body of the Theotokos.

The Holy Scriptures tell us that when our Lord was dying on the Cross, He saw His mother and His disciple John and said to the Virgin Mary, "Woman, behold your son!" and to John, "Behold your mother!" (John 19:25-27). From that hour, the Apostle took care of the Theotokos in his own home.
Along with the biblical reference in Acts 1:14 that confirms that the Virgin Mary was with the Holy Apostles on the day of Pentecost, the tradition of the Church holds that she remained in the home of the Apostle John in Jerusalem, continuing a ministry in word and deed.

At the time of her death, the disciples of our Lord who were preaching throughout the world returned to Jerusalem to see the Theotokos. Except for the Apostle Thomas, all of them including the Apostle Paul were gathered together at her bedside. At the moment of her death, Jesus Christ himself descended and carried her soul into heaven.

Following her repose, the body of the Theotokos was taken in procession and laid in a tomb near the Garden of Gethsemane. When the Apostle Thomas arrived three days after her repose and desired to see her body, the tomb was found to be empty. The bodily assumption of the Theotokos was confirmed by the message of an angel and by her appearance to the Apostles.

Source: goarch.org