The liturgy of the Church celebrates but one mystery: the life, death, and resurrection of Jesus Christ.
Each Sunday is the weekly commemoration of that mystery of Christ. Christians gather each Sunday to celebrate, in word and sacrament, their participation in Christ. The Lord's Day is consequently given primacy over other commemorations.
Each year the weekly commemoration is celebrated with particular joy when the Church keeps Passover or Easter. This observance includes forty days of preparation in Lent and fifty days of celebration in the Easter season. Easter is the central festival of the Church Year.
The commemoration of the birth of Jesus Christ at Christmas provides the focus for the other seasons of the Church Year. This festival, much later in origin than Easter, is associated with the Epiphany in some parts of the Christian world. Advent is the period of preparation for this celebration.