Unlike the Mass (which is a single occurrence, although repeatable throughout the day), the Office consists of multiple sessions of prayer at set hours during the day. Also, unlike the Mass which can be traced to direct institution by Christ, the Office has its origins in more generalized directions for continuous and watchful prayer (e.g., Ps. 119:164, Mark 13:34-35, Luke 11:2-4, 1 Thess. 5:17), which derive from the Jewish tradition of community prayer in the synagogue at specific times during the day. As learned from the Jewish custom, these prayers were at first only for the opening and closing hours of the day. Gradually, other fixed or “canonical” hours were added, and the final form was established by St. Benedict of Nursia in the first half of the 6th century. Benedict’s Rule lays out very clearly the times and order of daily prayer, including recitation of the entire psalter over the course of the week. The Rule also specifies readings from scriptural and patristic texts at Matins. Since monks were responsible for the running of Rome’s churches during this and the following century, the monastic Office came to form the basis also for the secular Office. These two uses, monastic and secular, remained the basis for daily prayer in the Western church throughout the Middle Ages, although - as one would expect - with considerable local and seasonal variation.
As originated by Benedict, the Office would occupy some four to five hours of a monk’s day; with gradual and sometimes intense elaboration, the daily Office at one point grew to where it was absorbing an astonishing ten to twelve hours, especially on the most important feasts. Reform of the Office was, obviously, a frequent refrain in those orders who split away from traditional Benedictine monasticism.
The eight services, with a highly simplified outline of their times (which would have varied according to local custom, as well as geographical and seasonal determination of the amount of daylight available):