Introduction and Pre-Monastic Office
The origins of the Divine Office, the Opus Dei, or "Work of God," are in the early Church. No exact date, however can be assigned to the inauguration of the official 'Prayer of the Church,' the Divine Office, as we've come to know it. Many scholars hold that the origins of the Christian Office are within the Jewish liturgical tradition, due to the fact that the Book of Psalms is the bulk of the structure for both Jewish and Christian public prayer (outside of the Mass). Jews and Christians prayed psalms and other Scripture texts especially in the morning and evening, and continue to do so. The connection between Jewish and Christian traditions seems clear, as Christ's first followers were Jewish and comfortable no doubt with existing forms of public prayer.
Along with possible Jewish roots, another important source of the Divine Office is the New Testament injunction to "pray without ceasing," found in Saint Paul's First Letter to the Thessalonians, a document that even pre-dates the written Gospels. First Thessalonians, chapter 5, verse 17 seems to have encouraged early Christians to gather regularly for prayer, following the example of the Apostles and "Mary the mother of Jesus" as described in the Acts of the Apostles (Acts 1:14). These gatherings of early Christians would have been in the morning and evening, but likely too at midmorning, midday, mid-afternoon; that is, the third, sixth and ninth hours, with the "zero hour" of each day being understood as approximately 6:00 am. Hence, Terce, Sext and None, as they eventually came to be known, corresponded to the third, sixth and ninth hours of the day, that is, three, six and nine hours after 6:00 am.
Prime, the first hour, was a later innovation in the West, beginning close to the time of Saint Benedict (480-547) in the sixth century. Because of its late beginning it is not universally accepted as an authentic part of the Divine Office, so is considered a disputed office. Prime was known to John Cassian, however, in the Egyptian desert at an earlier date (Inst. 3.4). Cassian claims the monks in Bethlehem invented Prime to keep monks from returning to bed between Lauds and Terce! That sounds reasonable enough!
For the early Christians, morning and evening prayers were the principal or major hours, as in the Jewish tradition. These two gatherings were the longer hours, meaning they were composed of more psalms and other sacred texts. The "little hours" of Terce, Sext and None were and are just that: shorter prayer times with fewer psalms and other texts.
Besides the morning and evening prayers, with minor or "little hours" in between, the early Christian liturgy also had an all-night Vigil service before the great feasts of the Church calendar, such as Easter, Pentecost and Christmas. The long Vigil was typically followed by the celebration of the Holy Eucharist at dawn, and then time allotted for a meal and rest. This structure is carefully maintained by many Orthodox monasteries today.
The Vigil for great feasts in some Orthodox monasteries today begins the night before, around 8:30 P.M., and only ends next morning around 7:00 A.M. with the conclusion of Mass! The only break might be for the main group of cantors, perhaps 7 or 8 monks, to go out for some coffee around midnight, during which time a soloist continues with the chanting. This sort of structure I experienced in the flourishing monasteries on Mount Athos in Greece in 1979, during a forty-day stay. I was in my 20s then, and the all-night Vigil was most enjoyable. I might feel less enthusiastic about the idea now! But I'm sure the monks there carry on the custom with vigor.
In any case, we have the early Christian witnesses of people like the pilgrim Egeria to the Holy Land around 380 A.D., coming perhaps from Spain, partaking in the all-night Christian Vigil in Jerusalem. The Jewish custom of the new day beginning at sundown is seen in this practice, which we still maintain in our Catholic tradition on Saturday nights--at First Vespers--as well as on the evening before Solemnities. In the Jewish tradition the new day always begins at nightfall. For us, it is only on Solemnities and Sundays. This is certainly a way to mark off the great feasts with practices different from those observed on regular or ferial days.
Some scholars of Church liturgy connect the origins of the night Vigil to the Book of Exodus account of the Hebrews fleeing Egypt at nighttime. The resurrection of Christ also occurred in the night. The modern practice of early morning Vigil, which Saint Benedict expected his monks to maintain, finds its origin as well in "waiting for the risen Lord" to come to us at dawn, as did the women disciples of our Lord who went to the tomb at daybreak. The idea of waiting is linked to being always ready to open the door of the heart, of one's time and all one has, should Jesus return even at midnight or before sunrise. To wait in the dark, keeping vigil, is a very ancient monastic custom, still preserved by stricter Orders and some individual monasteries such as Christ in the Desert.
As already stated, the Book of Psalms has always been the chief text used in whatever Christian Office is celebrated. Many of the psalms are chosen specifically to reflect the time of day of the Office--nighttime, morning, midday, etc. Other psalms would have a difficult time fitting into any particular time of day, but worthy material--as divinely inspired texts--for general use in the Divine Office. Some psalms in fact quite naturally develop a relationship with a particular feast or solemnity. Think of Psalm 2, for example, in relation to the birth of our Lord: "The Lord said to me: 'You are my Son. It is I who have begotten you this day.'" Or Psalm 71 for the same feast: "O God, give your judgment to the king, to a king's son your justice...The kings of Sheba and Seba shall bring him gifts." This especially calls to mind our Lord's Epiphany.
These and other appropriate psalms understandably are assigned for the Office during the Christmas season. Psalm 44 has a particular Marian sound and is assigned to feasts of our Lady. "Listen, O daughter, give ear to my words: forget your own people and your father's house."
The Christian community has always tried to look at the psalms in the light of the Risen Christ, and to find the deepest meaning of the psalms in Christ's saving deeds. In the earliest practice of celebrating the Divine Office, the laity, in congregations that we now call parishes, were the main participants. They would gather with their clergy--priests or deacons--and in unison or with soloists sing some or all of the Offices. They also made use of sacred images, candles, bells and incense to enhance the celebrations. Gestures of bowing, prostrating in prayer, crossing oneself, etc., were also part of the liturgy.