Theophany (from Greek theophania, meaning "appearance of God") is one of the Great Feasts of the Orthodox Church, celebrated on January 6. It 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).
Baptism of Christ
This observance commemorates Christ's baptism by John the Forerunner in the River Jordan, and the beginning of Christ's earthly ministry. The Feast of Theophany is the culmination of the Christmas Season, which starts on December 25 and ends on January 6. In mystic commemoration of this event, the Great Blessing of Water is performed on this day, and the holy water so blessed is used by the local priest to bless the homes of the faithful.
The feast is called Theophany because at the baptism of Christ the Holy Trinity appeared clearly to mankind for the first time -- the Father's voice is heard from Heaven, the Son of God is incarnate and standing physically in the Jordan, and the Holy Spirit descends on Him in the form of a dove.
This feast is also sometimes referred to as Epiphany by English-speaking Orthodox Christians, 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.
Originally, there was just one Christian feast of the shining forth of God to the world in the human form of Jesus of Nazareth. It included the celebration of Christ's birth, the adoration of the Wisemen, and all of the childhood events of Christ such as his circumcision and presentation to the temple as well as his baptism by John in the Jordan. There seems to be little doubt that this feast, like Easter and Pentecost, was understood as the fulfillment of a previous Jewish festival, in this case the Feast of Lights.
(Troparion - Tone 1) 
- When You, O Lord were baptized in the Jordan
- The worship of the Trinity was made manifest
- For the voice of the Father bore witness to You
- And called You His beloved Son.
- And the Spirit, in the form of a dove,
- Confirmed the truthfulness of His word.
- O Christ, our God, You have revealed Yourself
- And have enlightened the world, glory to You!
(Kontakion - Tone 4)
- Today You have shown forth to the world, O Lord,
- and the light of Your countenance has been marked on us.
- Knowing You, we sing Your praises.
- You have come and revealed Yourself,
- O unapproachable Light.