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
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.
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.
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.