Catechism of the Catholic Church §1174 — excerpt
The mystery of Christ, his Incarnation and Passover, which we celebrate in the Eucharist especially at the Sunday assembly, permeates and transfigures the time of each day, through the celebration of the Liturgy of the Hours, "the divine office."
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.
Also cites: SC, Ch. IV, 83-101 · SC 84 · SC 98