In the Catholic Church, the new year starts on the First Sunday of Advent--this is the year we change reading cycles and when the circular liturgical calendar starts anew. (A "reading cycle" is one of the three Sunday cycles of readings--A, B, or C. Year A focuses on the Gospel of Matthew, Year B on the Gospel of Mark, and Year C on the Gospel of Luke. The Gospel of John is sprinkled throughout all the cycles, and is always the Gospel for Good Friday, for example. The daily lectionary [for daily Mass] goes on a two year cycle.) 

The seasons of the Liturgical Year are: 

1. Advent
2. Christmastime
3. Ordinary Time
4. Lent
5. The Triddum-- Holy Thursday, Good Friday, and Holy Saturday
6. Easter
7. Ordinary Time
8. Advent 

Each season has its particular colors and emphases. Advent, for example, has a penitential flavor, but it's not as severe in Lent--it's more a sense of preparing joyfully for the coming of Christ in the Incarnation. What can we do to make ourselves ready for His appearance? Lent has more of the penance we think of as penance--being sorry for our sins, giving things up, etc. Advent's penance is slightly different, even though both seasons are purple in liturgical color, and purple stands for penance in church parlance. 

The four weeks of Advent  are broken into three "purple" and one "rose" week--the rose vestments and candles are to remind us to "rejoice" as St. Paul tells us in the second reading of the Third Sunday of Advent. It's called "Gaudete" (rejoice!) Sunday. ("Rejoice in the Lord always! I say it again: rejoice!") 

In Advent, we focus on preparing ourselves for both Jesus' first coming--in the Nativity--and his Second coming at the end of the world. How can we live Advent well? There are tons of books written about that. But a key thing is to remember that it is a time of preparation--it's NOT Christmas. The tree shouldn't go up on Dec. 1 and come down on the 26th. The Christmas season, in the Church, lasts from Christmas Day until at least the Baptism of the Lord in January--and in some churches, like mine, the old traditions are upheld, where there are forty days of Christmas, ending on Candlemas (Feb. 2), which is when our parish creche is put away. I love this tradition and I've adopted it in my own house. But however long you celebrate Christmas, remember that Advent and Christmas are two distinct seasons.