The establishment of the Nativity Fast, as well as other multi-day fasts relates to the ancient times of Christianity. But already from IV century St. Ambrosius of Milan, Philistrius and St. Augustin mention the Nativity Fast in their works. In V century Leo the Great wrote about the Nativity Fast . At first the Nativity Fast lasted seven days for Christians, and for the others - a bit longer. At the Council of 1166 when was held in the time of the Constantinople Patriarch Luke and Byzantine emperor Manuel a forty-day fast was ordered to be observed by all Christians before the great feast of the Nativity of Christ.
The Nativity Fast is the last multi-day fasting period in the year. It starts on November 15/28 and lasts until December 25/ January 7, it lasts forty days as well as the Great Lent. Since the Fast starts on the feast day of St. Apostle Phillip (November 14 acc. to the Julian calendar), it's also called the Phillip's Fast.
For us, the Nativity Fast serves to refresh the last part of the year - mystically renewing our spiritual unity with God and preparing us for the Feast of the Nativity of Christ.
St. Leo the Great wrote: "Four periods [of the year] have been set aside as times of abstinence, so that over the course of the year we might recognize that we are constantly in need of purification, and that amid life’s distractions, we should always strive by means of fasting and acts of charity to extirpate sin, sin which is multiplied in our transitory flesh and in our impure desires."
According to Leo the Great, the Nativity Fast is a sacrifice to God in return for the gathered harvest. The Holy Hierarch stated, "Just as the Lord has generously granted us abundance of the fruits of the earth, so should we, during the time of this Fast, be generous to the poor."
According to St. Simeon of Thessalonica, "…the Nativity Forty-day Fast represents the fast undertaken by Moses, who, having fasted for forty days and forty nights, received the Commandments of God, written on stone tablets [of the Law]. And we, fasting for forty days, will reflect upon and receive from the Virgin of the living Word - not written upon stone, but born, incarnate, and we will commune of His Divine Body."
The Nativity Fast was established to allow us through repentance, prayer and fasting to cleanse ourselves before the Nativity of Christ, so that with clean heart, soul, and body, we might reverently meet the Son of God, Who has come into the world and so that in addition to bringing the usual gifts and sacrifices, we might bring Him our clean hearts and a desire to follow His teachings.
May God grant that we all spend the salvific days of the Nativity Fast in such a way as to be a great benefit to our souls.