In the Catholic Church (the same today as in medieval times), the day is divided into hours based on divisions of three hours apiece. These hours of Prime, Terce, Sext, None, Vespers, and the night Vigils are based on divisions established in Roman times. The night Vigils consist of Matins and Lauds, with Matins corresponding to three different times — nine o’clock at night, midnight, and three o’clock in the morning — and Lauds being recited at dawn. The offices of the day correspond to the following times: Prime (6 A.M.), Terce (9 A.M.), Sext (midday), None (3 P.M.), Vespers (6 P.M.). Compline, which is not included in the list above, is recited at nightfall. It is not included in the arrangement as its origin falls at a later date.
The division of hours does not go back to the early dates of Christianity. It seems from historical record that there was no official or public prayer at that time, except for the Eucharist service (Lord’s Supper) or vigils, which Matins and Lauds most likely represent, and these vigils most likely included the chanting of psalms and of readings from Holy Scripture, the Law, and the Prophets, the Gospels and Epistles, and a homily. According to Jewish tradition, the equivalent offices of Terce, Sext, None, and Vespers were already practiced, and the later Christians adopted these into their own services.
At first, these offices were only meant for private prayer, but as the Church grew and monasteries began to take root, flourish and gain influence (around the turn of the 4th century), the canonical hours became the standard for public prayer. In the second half of the fourth century, the document “Peregrinatio ad Loca Sancta,” written around AD 388, described the Liturgy followed by the Church of Jerusalem at that time.
The two offices of Compline (mentioned briefly above) and Prime were established later: Prime in the late 4th century and Compline most likely in the 6th century by St. Benedict. For Compline, however, it must be noted that while St. Benedict may have had a direct influence for this canonical hour in western medieval Europe, there already existed a prayer for the end of the day in other regions.
Cabrol, Fernand. “Breviary.” The Catholic Encyclopedia. Vol. 2. New York: Robert Appleton Company, 1907. 9 Sept. 2008