As contained in approved Breviaries, the group of psalms, hymns, prayers, readings from the Old and New Testaments, patristic homilies, and lives of saints, arranged and formulated by the Church, whereby daily public or liturgical prayer is offered to God. It is also called Cursus, Canonical Hours, or Opus Dei. It is the public and official prayer by which the Church, as a visible society and as the Mystical Body of Christ, offers in union with her Divine Founder, adoration and supplication to God. In consequence, the regulation of this official prayer depends upon the supreme authority in the Church who deputes certain representatives (the priesthood) to fulfill this obligation in the name of the universal Church. “It is the common prayer which is offered to God by the minister of the Church in the person of all the faithful” (Saint Thomas). It follows from these dogmatic principles that all the faithful habitually pray in the recitation of the Office and that a priest, even though he recite the Office privately, prays in the name of the entire Church.
The history of the Divine Office may be divided into three periods. The first period extending from the Apostolic age to the pontificate of Gregory I (6th century) witnesses the formation of the fundamental parts. From Apostolic days, the Church dedicated certain fixed hours to public prayer. These hours were comprised in the vigils extending from evening to dawn. From these vigils which were daily celebrated in the 4th century emerged the major hours of the Office, Vespers, Matins, and Lauds. In the same century the day hours of Terce (6 a.m.), Sext (12 m.), and None (3 p.m.), which previously had been hours of private prayer, had become a public fixed custom. The hour of Prime (6 a.m.) was instituted in the same century and the hour of Compline, in the 6th century. From the earliest days it was the Book of Psalms that furnished the groundwork of this public prayer. In the West Saint Benedict (6th century) rearranged systematic distribution of the psalms over the canonical hours and in other ways regulated the structure and content of the Office.
During the second period (6th to 16th century) the Roman Office as celebrated in the Roman basilicas spread into France, England, and Germany. To it were added the festivals of many saints. Through monastic influences, principally Franciscan, it received periodic additions; e.g., hymns (12th century), Gradual psalms, Suffrages, Offices of the B.V.M. and of the Dead, final antiphons.
The third period, from the 16th century to our own day, is characterized by the simplification of the Office and a rearrangement of the psalms to restore the traditional ideal of the recitation of the entire Psalter within the compass of a week. Each complete daily Office requires 33 psalms which are divided among the canonical hours; the longer psalms are divided into two or more parts. The Divine Office is intimately connected with the Sacrifice of the Mass which regulates the Office of the day. Within the compass of the annual ecclesiastical cycle, the Church commemorates and renews the mysteries of the life of Christ and the work of the Redemption and honors the saints of God. This fundamental idea determines the structural contents of the Office as assigned to the various canonical hours. As contained in the Breviary, the Office is divided into: rubrics or directions for the recitation of the Office; the Ordinary, or the normal framework of the Office; the Psalter, or the psalms assigned to each hour of each day; the Proper of the Season, or the prayers and current scriptural reading and patristic homilies; the Proper of the Saints, or the prayers and historical lessons for the Office of the saints; the Common of the Saints, or certain variable parts of the Office which may be used for many saints according to their classification; a Supplement, containing the Office of B.V.M., the Office of the Dead, Penitential psalms, litanies, etc.