Advent is often referred to as the beginning of the Church Year. The reason stems from the fact that the Advent readings are placed first in the lectionary. However, putting the Advent readings on page one was simply an arbitrary decision made centuries ago.
Advent could also be called the end of the Church Year, for its emphasis is preparation for the Second Coming of Christ at the end of time. Actually every day of the Church Year celebrates the whole life of Christ, the paschal mystery. Only the focus changes from the birth of Jesus to his public life to his death and resurrection. So every day is everything. Every day is Christmas. Every day is Good Friday. Every day is Lent, and every day is Advent. And every day is the beginning and the end. And the One whom we celebrate is the Alpha and the Omega, the Beginning and the End.