For one to understand the origins of Advent, it is important to trace briefly the origins of Christmas itself. In part, Christmas was an attempt to counter pagan festivities connected with the winter solstice in the time of the early church. In the western portion of the Roman Empire, the date for observing the birthday of the sun was December 25 (in the East, January 6). This date was set as early as 274, sixty-two years before there is any solid evidence that Christians in Rome celebrated this same date as Christmas. Indeed, it was during the reign of the Emperor Constantine in the 4th century that the Church, at Constantine's encouragement, adapted the celebration and practice of sun-worship into the celebration of the birth of Jesus. The popularity of this association was undoubtedly aided by the numerous Christological controversies of the fourth century and the need to challenge them (in particular, the need to refute Arianism, a school of thought which challenged the notion that Jesus was truly divine).
The season of Advent developed in the western portion of the Roman Empire, probably in 4th century Gaul. Originally, it was a three-week period of preparation before Epiphany, which was celebrated on January 6 (the celebration of Christ's nativity, his baptism, and his first miracle at Cana). A parallel development occurred in Rome, as Advent there grew out of a single day's fast before Christmas. Gradually, the preparatory period before Epiphany in Gaul combined with the pre-Christmas fasting motif of Rome, and Advent became a period of penitence and preparation before Christmas. In subsequent centuries, Advent incorporated the theme of the Second Coming in addition to the birth of Jesus. Pope Gregory the Great (6th century) was the first to fix its length at four-week, which is the same length of time which western Christianity observes in modern times.
Advent literally means "coming" or "arrival" in Latin. The season of Advent marks the beginning of the liturgical year in the western churches which observe the liturgical calendar. In modern times, Advent is a season of preparation, a time when Christians ready themselves both for the coming of the Messiah at Christmas and his return as Judge and Redeemer at the end of history. Indeed, leading up to Advent in the liturgical year is a growing emphasis on the consummation of history, which comes to a climax on the final Sunday of the liturgical year, a Sunday known as Christ the King. Following Christ the King Sunday is the first Sunday of Advent. On this Sunday, there is always an emphasis on the final parousia (the return of Jesus at the end of history). The second and third Sundays of Advent focus on the John the Baptist's promise of the Messiah's coming. The fourth Sunday of Advent tends to highlight the annunciation, when the angel, Gabriel, announces to Mary that she will bear a child who will be the Son of God. Advent concludes with the celebration of Jesus' actual birth on Christmas Eve and Christmas Day. Thus, in the four Sundays which constitute Advent, there is a marked shift from the expectation of the consummation of history to the preparation for the incarnation and nativity of Jesus the Messiah. As a result, it is in the season of Advent that the beginning and end times meet, and we as Christians remember that the roots and the destiny of human history is firmly embedded in the Sovereign One, whom we call Immanuel, "God with us."