The date of Christmas was fixed as December 25 by Pope Julius I in the fourth century. This date was probably chosen for several reasons. By fixing Christmas on December, the twelve days from Christmas to Epiphany provided a Christian alternative to the bawdy Roman Saturnalia festival (December 17 to 23)—allowing Christians to celebrate around the same time but separately from pagans. Another factor was likely that December 25 was already a Roman holiday, the feast of Sol Invictus (The Unconquered Sun), a solstice celebration. According to the messianic prophecy of Malachi 4:2, this could be given a Christian reinterpreted as a feast of the "sun of righteousness, rising with healing in his wings." Finally, in the fourth century it was reckoned (incorrectly) that Passover fell on March 25. It was believed that Jesus died on the anniversary of his conception (hence the date of the Annunciation), and it was assumed that Christmas should fall exactly nine months after the Annunciation.
The pagan Roman festival of Saturnalia was celebrated from December 17 to 23. The festival was characterized by role reversal (e.g. slaves commanding their masters), gambling (ordinarily not allowed), gift-giving, the suspension of criminal sentencing and war, and sacrifice (of animals and in the form of gladitorial combat).
These associations were later harmonized with the Christian mysteries. Role reversal reminded Christians of the Incarnation, in which God becomes Man in order to lead humankind back to God. Gift-giving harkened back to the gifts of magi and was practiced among Christians. The reprieve of criminal punishment and war came to symbolize the reign of Christ, the Prince of Peace. Sacrifice, of course, reminded Christians of their penitential practices and of Jesus himself.
The Talmud gives a Jewish interpretation of Saturnalia, saying that it goes back to Adam. Seeing that the days were growing shorter before the winter solstice, Adam was afraid the world was going to be destroyed as a punishment for sin, so he fasted for eight days. Seeing that the days were getting longer again after the solistice, he feasted for eight days.
Medieval Christian practice tracked this Jewish interpretation: the eight days immediately preceding Christmas (from December 17) were marked by an intensification of the fasting and prayer immediately before the celebration and festivity of Christmas.
The Great O Antiphons, written by Benedictines around the sixth century, are the antiphons used before the Magnificat at Evening Prayer (Vespers) from December 17 to 23, the last seven days of Advent. Since Vatican II, they are also used as the Alleluia verses at Mass on those days.
They are refered to as O Antiphons because the first word of each antiphon is addressed to the Messiah with an expressive "O" followed by one of the titles given to the Messiah by the Old Testament prophecies:
In the Middle Ages, Saturnalia-like revelry accompanied the twelve days of Christmas—abundant feasting, drinking, games, and gift-giving. Although many Church authorities tried to temper the celebration of a "Feast of Fools," in France, on December 28 (the Feast of the Holy Innocents), a boy would be elected bishop for a day, harkening back to the role reversal of Saturnalia.
This boy-bishop, called the Saturnalia princeps (Prince of Saturnalia), was vested in full episcopal vestments and presided over certain rites. In some dioceses, the bishops themselves completed the reversal by performing the choirboy's usual liturgical ministry. There is obviously a fun (and funny) element to this tradition; but it also celebrated the mystery of the Incarnation: "He has cast down the mighty from their thrones, and has lifted up the lowly" (Luke 1:52).
The custom of electing a child-bishop has been revived in many English (Anglican) cathedrals and also some Episcopal churches in the United States. In the Catholic world, it has been revived at Westminster Cathedral, London, by a Catholic boys boarding school in France, and by several cathedrals in Spain, where the children's choir also vest as cathedral canons.
Below is a photo 1927 of the boy-bishop and boy-canons from the Benedictine abbey of Santa Maria de Monserrat in Catalonia, Spain. Google "boy bishop" for some interesting photos and articles!