The following article is one which we shared back in 2006, and comes from byzcath.org.
The Preparatory Season for the Nativity of Our Lord - The Phillipian Fast
Rev. Msgr. Russell A. Duker
Archdiocese of Pittsburgh
The oldest Christian feast is the Resurrection of our Lord (Pascha). This Holy Day includes a whole cycle of feasts such as the Ascension and Pentecost. It is the great feast of our redemption and sanctification. Later Holy Days followed slowly until the fourth century. After the Church won official recognition and full freedom of worship and evangelization, our present calendar of festal celebration began to develop. This development was motivated by the Church's desire to honor both the events in the life our our Lord and the memory of the holy martyrs. Eventually the Church established a full year Christian calendar.
We are familiar with the preparatory period before the Resurrection. This is the "Great Fast" or the "Holy Forty Days' Fast". The celebration of the birth of our Lord cannot be ascertained before the middle of the fourth century. The Church at Rome was the first to celebrate our Lord's birth. Many think that the date of December 25 was chosen to supplant the feast of the god Mithra and the solemn celebration of the birth of the invincible sun god. Others think that the date was chosen for the same reason that the Roman pagans honor the victory of the sun. It is around this date that the sun overcomes the darkness and the days become longer. Several times the prophets call Jesus Christ "Sun of Justice." It was deemed proper to choose the day when the sun begins its victorious cycle of light by shortening the duration of the night.
According to some sermons of St. Gregory of Nazianzus, he introduced this feast into the Eastern Church about the year 379 or 388. After his departure from Constantinople the celebration of Christ's Nativity on December 25 was neglected. In 395 Emperor Honorius reinstituted the celebration. St. John Chrysostom tells us how he introduced this feast at Antioch sometime around 380. He explicitly says how he introduced it in imitation of the Church at Rome. St. John believed that the Roman Christians knew the date of Christ's birth better than anybody else since the imperial city archives were accessible to them.
The first mention of a preparatory period before Christmas is mentioned in a decree of the Council of Saragossa (380). The Council Fathers stated that every Christian should daily go to church from December 17 until the Theophany (January 6th). At the Synod of Mac (581) in present day France it was decreed that from November 11, the day of St. Martin, until December 24 every Christian should fast 3 times a week (Monday, Wednesday, Friday).
Our pre-Nativity period of preparation developed rather late. Scholars do not agree about the exact time it began. Some hold that it began in the sixth century. Others believe it began in the seventh or eighth century. The present liturgical pre-Nativity season was finally established at the Council of Constantinople (1166). The Council decreed that the fast would begin on November 15 and last until December 24 inclusive. Thus, there was created another 40 day fast.
The pre-Nativity fast is often called "Phillip's Fast" because it begins on the day after the feast of St. Phillip. The fast was introduced to prepare the Church for a worthy celebration of the great and holy day of the Birth of Christ. The regulations for the fast were far more lenient than the Great Fast before Pascha. Only Monday, Wednesday, and Friday were days of strict fasting without meat, dairy products or oil (in Slavic countries). On Sundays fish was permitted. Lay people were at first permitted to eat fish on other days, too, until the monastic rigoristic influence prevailed. It is interesting to observe that the famous 12th century Byzantine canonist Balsamon expressed the opinion that it would be enough if the lay people fasted only one week before Christmas. In 1958 a modern Greek author, Christos M. Enislides, welcomes Balsamon's suggestion and believes that the best solution would be for the Church at large to abstain from meat and dairy products for 33 days. During the last seven days of the fast everybody should observe the strict fast.
To worthily meet our Lord and Savior, we should sanctify this pre-Nativity season of the Phillipian Fast. Sanctifying means spending our time in faith and in the service of God and in kindness towards our neighbor, especially those who are in need of our assistance. And we should think of what we would have been had Christ not come to our lowliness and poverty. Together with the whole of the Byzantine Church we should try to meet Christ as he deserves to be met and as it will, in His mercy, best serve our spiritual benefit!