From Orthodox Wiki:
Great and Holy Friday begins with reading of the Royal Hours leading up to Vespers of Friday afternoon, at which time the removal of the Body of Christ from the Cross is commemorated. The priest removes the Body of Christ, the Epitaphios, from the Cross, wraps it in a white cloth and carries it into the altar. In an evening service, called the Lamentations at the Tomb, the priest carries the Epitaphios, the painted or embroidered cloth representation of Christ, from the altar around the church before placing it in the Sepulcher, a bier symbolizing the Tomb of Christ. This procession, with the faithful carrying lighted candles, represents Christ's descent into Hades.
Great and Holy Saturday Vespers and a Divine Liturgy of St. Basil are served, marked with readings of Psalms and Resurrection hymns that tell of Christ's descent into Hades, celebrated as the "First Resurrection" of Adam and the conquering of Death. It is appointed by the typikon to be celebrated in the afternoon, though it is served in the morning in many places.
This service comes from the ancient liturgical tradition of the Church of Constantinople and was its primary Paschal service. The hymn "Arise O God" from the Psalms was the original primary Paschal hymn before "Christ is risen" came to take its place. Its place as the ancient Constantinopolitan Paschal celebration is what gives the service such a bright and resurrectional character.