The Prefeast Vespers service is in preparation for Theophany, one of the 12 Great Feasts of the Church.
Theophany is the Feast which reveals the Most Holy Trinity to the world through the Baptism of the Lord (Mt.3:13-17; Mark 1:9-11; Luke 3:21-22). God the Father spoke from Heaven about the Son, the Son was baptized by the St John the Forerunner, and the Holy Spirit descended upon the Son in the form of a dove. This feast is also sometimes referred to as Epiphany but that name more properly refers to the Western Christian feast falling on that same day and commemorating the visit of the Magi to the child Jesus. From ancient times this Feast was called the Day of Illumination and the Feast of Lights, since God is Light and has appeared to illumine “those who sat in darkness,” and “in the region of the shadow of death” (Mt.4:16), and to save the fallen race of mankind by grace.