Why does the liturgical year start on Advent, and yet the readings on the first Sunday of Advent are about the end of the world? Well, blame the choirs and the scribes who painstakingly copied the choral tomes.
The liturgical year originally began on Christmas Day. The Incarnation signified the onset of our salvation, and Holy Mother Church cemented the significance of this temporal fulcrum by starting the cycle of Her worship with the Liturgy of the Incarnation of the Lord, and ending it with the Liturgy reminding man of the end of days, of the second coming of Christ, the Second Advent of the Lord.
Enter cantors and their scribes. The scribes started compiling Church music, and, in so doing, placed the chants of Advent before the chants of Christmas. The altar books and the choir books diverged for some time, until, eventually, the altar books yielded to the arrangement of the choir books. Henceforth, Advent became the start of the liturgical year of the Church.
Ut in omnibus laudetur Dominus.