Advent has probably been observed since the fourth century. Originally, it was a time when Christians readied themselves for baptism. During the Middle Ages, Advent became associated with preparation for the Second Coming. In early days Advent lasted from November 11, the feast of St. Martin, until Christmas Day. Advent was considered a pre-Christmas season of Lent when Christians devoted themselves to prayer and fasting. The Orthodox Eastern Church observes a similar Lenten season, from November 15 until Christmas, rather than Advent.
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.
Advent is a creation of the Western churches that looked to Rome as their leader. There were two main streams flowing into it. The first came out of France, during the fourth century AD, probably from Celtic monks. A period of about six weeks before Christ's Mass was used as a penitential and devotional period, a lesser Lent. The second stream came from Rome, where there was a practice of having a three-to-six week fast during which you had to come to church regularly. This was a fast before the feast of Christmas time.
The current form of Advent crystallized under Pope Gregory I, who set the current four-week length, and wrote liturgical materials for use in Advent. By the 10th century, the Celtic 'get ready' prayers and practices had been fully brought into the Roman form. Later on, the church adopted a system of liturgical colors, and Advent received a purple color not unlike Lent's. The 20th century brought a rediscovery of joy in Advent preparations; this was signaled among Protestants by using the color blue (with or without a touch of red in it).
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.