Vespers is the principal evening worship of the Christian Church. As celebrated in the Byzantine Rite, it serves as the beginning of the liturgical day, and has as its principal theme a commemoration of Christ, the never-setting Light of the world.
Depending on local tradition or circumstances, Vespers is normally celebrated between 4 pm and 8 pm; ideally it is celebrated so that the lamps in the church are lit as darkness falls and the Hymn of the Evening (“O Joyful Light”) is sung.
Combining Vespers with Other Services
Vespers may be combined with the Divine Liturgy on the eves of Nativity and Theophany, and on the Feast of the Annunciation, according to church tradition. The Old Testament readings of Vespers are followed by a Small Litany, and the Divine Liturgy continues with the singing of “Holy God”.
During the Great Fast, Vespers is combined with a solemn service of Holy Communion to form the Divine Liturgy of the Presanctified Gifts.
Adapted from: https://mci.archpitt.org/liturgy/Vespers,html