As explained in greater detail in the Catechism of the Catholic Church, the liturgy is something that "the whole Christ", Head and Body, celebrates — Christ, the one High Priest, together with his Body, the Church in heaven and on earth. Involved in the heavenly liturgy are the angels and the saints of the Old Covenant and the New, in particular Mary, the Mother of God, the Apostles, the Martyrs and "a great multitude, which no man could number, out of every nation and of all tribes and peoples and tongues" (Revelation 7:9). The Church on earth, "a royal priesthood" (1 Peter 2:9), celebrates the liturgy in union with these: the baptized offering themselves as a spiritual sacrifice, the ordained ministers celebrating at the service of all the members of the Church in accordance with the order received, and bishops and priests acting in the person of Christ.

The Catholic liturgy uses signs and symbols whose significance, based on nature or culture, has been made more precise through Old Testament events and has been fully revealed in the person and life of Christ. Some of these signs and symbols come from the world of creation (light, water, fire, bread, wine, oil), others from life in society (washing, anointing, breaking bread), others from Old Testament sacred history (the Passover rite, sacrifices, laying on of hands, consecrating persons and objects).

These signs are closely linked with words. Though in a sense the signs speak for themselves, they need to be accompanied and vivified by the spoken word. Taken together, word and action indicate what the rite signifies and effects.

Sunday, which commemorates the resurrection of Christ and has been celebrated by Christians from the earliest times (1 Corinthians 16:2; Revelation 1:10; Ignatius of Antioch: Magn.9:1; Justin Martyr: I Apology 67:5), is the outstanding occasion for the liturgy; but no day, not even any hour, is excluded from celebrating the liturgy. The sole exception is for the Eucharistic liturgy on Good Friday and on Holy Saturday before the Easter Vigil, when it is not celebrated.

The Liturgy of the Hours consecrates to God the whole course of day and night. Lauds and Vespers (morning and evening prayer) are the principal hours. To these are added one or three intermediate prayer periods (traditionally called Terce, Sext and None), another prayer period to end the day (Compline), and a special prayer period called the Office of Readings (formerly known as Matins) at no fixed time, devoted chiefly to readings from the Scriptures and ecclesiastical writers. The Second Vatican Council suppressed an additional 'hour' called Prime. The prayers of the Liturgy of the Hours consist principally of the Psalter or Book of Psalms. Like the Mass, the Liturgy of the Hours has inspired great musical compositions. An earlier name for the Liturgy of the Hours and for the books that contained the texts was the Divine Office (a name still used as the title of one English translation), the Book of Hours, and the Breviary. Bishops, priests, deacons and members of religious institutes are obliged to pray at least some parts of the Liturgy of the Hours daily.