What is Advent? Glad You Asked.

Advent (Lat. ad-venio, to come to), according to present usage, is a period beginning with the Sunday nearest to the feast of St. Andrew the Apostle (30 November) and embracing four Sundays. The first Sunday may be as early as 27 November, and then Advent has twenty-eight days, or as late as 3 December, giving the season only twenty-one days. With Advent the ecclesiastical year begins in the Western churches.

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.

from The Catholic Encyclopedia, 1913

24 Hours That Changed the World : An Advent Study for Youth