Catechism of the Catholic Church §1195 — excerpt
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.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.