Once more, it is the start of a new liturgical year, and time to swap breviary volumes! You can find an overview of the changes to the Office for Advent here.
The Catholic Encyclopedia notes that "During this time the faithful are admonished:
•to prepare themselves worthily to celebrate the anniversary of the Lord's coming into the world as the incarnate God of love,
•thus to make their souls fitting abodes for the Redeemer coming in Holy Communion and through grace, and
•thereby to make themselves ready for His final coming as judge, at death and at the end of the world.
At Matins the readings start on the Book of Isaiah, but all of the texts for I Vespers and Sunday are proper for the day, and draw out the nature of the new liturgical season.