By keeping the memorials of the saints - first of all the holy Mother of God, then the apostles, the martyrs, and other saints - on fixed days of the liturgical year, the Church on earth shows that she is united with the liturgy of heaven. She gives glory to Christ for having accomplished his salvation in his glorified members; their example encourages her on her way to the Father.
Source: Catechism of the Catholic Church (Vatican.va)