History of Advent
It cannot be determined with any degree of certainty when the celebration of Advent was first introduced into the Church. The preparation for the feast of the Nativity of Our Lord was not held before the feast itself existed.
One of the earliest references to Christmas being celebrated on December 25 appeared in Antioch in the middle of the second century. At that time, Christians were still persecuted. An official determination was made in the fourth century, when the Roman emperor Constantine embraced Christianity, thereby ensuring the legality of Christmas celebrations. The Council of Tours in 567 established the period of Advent as a time of fasting before Christmas. They also proclaimed the twelve days from Christmas to Epiphany a sacred, festive season.
According to present usage , Advent is a period beginning with the Sunday nearest to the feast of St. Andrew the Apostle (30 November) and embracing four Sundays. The first Sunday may be as early as November 27th, and then Advent has twenty-eight days, or as late as December 3rd, giving the season only twenty-one days.
The popular idea that the four weeks of Advent symbolize the four thousand years of darkness in which the world was enveloped before the coming of Christ finds no confirmation in the Liturgy.
The familiar carol “O Come, O Come, Emmanuel” belongs to the Advent season since it celebrates the expectation of Christ’s coming rather than His actual birth.