Theophany is the Feast which reveals the Most Holy Trinity to the world through the Baptism of the Lord. 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. 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" , and to save the fallen race of mankind by grace. In the ancient Church it was the custom to baptize catechumens at the Vespers of Theophany, so that Baptism also is revealed as the spiritual illumination of mankind. The origin of the Feast of Theophany goes back to Apostolic times, and it is mentioned in The Apostolic Constitutions. From the second century we have the testimony of St Clement of Alexandria concerning the celebration of the Baptism of the Lord, and the night vigil before this Feast. On the Feast of the Baptism of Christ, the Holy Church proclaims our faith in the most sublime mystery, incomprehensible to human intellect, of one God in three Persons. It teaches us to confess and glorify the Holy Trinity, one in Essence and Indivisible. It exposes and overthrows the errors of ancient teachings which attempted to explain the Creator of the world by reason, and in human terms. The Church shows the necessity of Baptism for believers in Christ, and it inspires us with a sense of deep gratitude for the illumination and purification of our sinful nature. The Church teaches that our salvation and cleansing from sin is possible only by the power of the grace of the Holy Spirit, therefore it is necessary to preserve worthily these gifts of the grace of holy Baptism, keeping clean this priceless garb, for "As many as have been baptized into Christ, have put on Christ". On the day of Theophany, all foods are permitted, even if the Feast falls on a Wednesday or Friday.
Source: Orthodox calendar