In the time leading up to Christmas, the Catholic Church celebrates advent. Advent is a necessary part of the Christmas season because it is the time when Catholics repent for their sins and perform penance. They want to be as pure as they can be when Christmas comes, in order to receive the gift of God’s presence at the Christmas Mass of Holy Eucharist. During Advent, there are four candles that are lit each week before Christmas.
In addition, the Church teaches that:
[w]hen the Church celebrates the liturgy of Advent each year, she makes present this ancient expectancy of the Messiah, for by sharing in the long preparation for the Saviors first coming, the faithful renew their ardent desire for his second coming. By celebrating [John the Baptists] birth and martyrdom, the Church unites herself to his desire: He must increase, but I must decrease (Catechism, no. 524; original emphasis).
On Christmas day, families attend church and receive the Holy Eucharist. They celebrate the birth of Christ and the church is typically decorated with a nativity scene and other decorations, in stark contrast to the decoration-less advent season.