What Christian holiday, commemorating the manifestation of God in human form, is celebrated by Greek Orthodox churches using the Julian calendar?

The Christian holiday commemorating the manifestation of God in human form, as celebrated by Greek Orthodox churches using the Julian calendar, is known as Epiphany, or more commonly in the East, Theophany. This significant feast day marks the baptism of Jesus Christ in the Jordan River by John the Baptist. It is during this event that the Holy Trinity—God the Father, God the Son, and God the Holy Spirit—is revealed, signifying the divine nature of Jesus to the world. The name "Epiphany" itself comes from the Greek word "epiphaneia," meaning "appearance" or "manifestation," directly reflecting this revelation.

While many Western Christian churches celebrate Epiphany on January 6th, focusing primarily on the visit of the Magi to the infant Jesus, Eastern Orthodox churches, including the Greek Orthodox, primarily commemorate Jesus' baptism. Due to their adherence to the Julian calendar, which is currently 13 days behind the Gregorian calendar, Greek Orthodox churches observe Epiphany on January 19th. This makes it one of the oldest and most important feasts in Eastern Christianity, often referred to as "Theophany" or "The Feast of Lights" (ta Phota), symbolizing the enlightenment of humanity by the Holy Trinity.

The celebration of Epiphany is rich in tradition, particularly the "Great Blessing of the Waters." On this day, a priest blesses a body of water—whether a sea, river, or lake—by throwing a cross into it. Young men often brave the cold waters to retrieve the cross, with the one who recovers it believed to be blessed for the entire year. This ritual not only commemorates Jesus' baptism but also symbolizes the purification and sanctification of the waters.