Sources for Medieval Liturgy
Liber Pontificalis (Book of Pontiffs)

The term Liber Pontificalis refers to a series of books edited throughout the Middle Ages listing the bishops of Rome and their principal accomplishments. Starting in the third century, the Liber was nothing more than a listing of the bishops' names. Sometime early in the sixth century, however, a redactor began adding biographical information to the lists. Papal scribes periodically updated the work adding the names and deeds of new popes. By the seventh century the entries had grown to be long and detailed biographies of their subjects.

The Liber Pontificalis is a rich source of information about liturgical practices in Rome. The following excerpt concerning the life of Pope Stephen II (752-757) tells us something about the celebration of the Night Office during his pontificate. Originally, the Night Office (Vigils or Matins) was a long service of readings, responses, and psalms celebrated between midnight and dawn. Clearly, by Pope Stephen's day, those responsible for celebrating this office had developed the custom of performting the vigil earlier in the evening in order to receive the benefit of a full night's sleep. Stephen reinstated the Night Office to its proper time and founded another monastery of monks (the fourth) near St. Peter's Basilica to ensure the correct celebration of this liturgy.

The lives of the popes found in the Liber Pontificalis through the ninth century have now been published in an excellent English translation with extensive notes in three volumes by Raymond Davis. The following is excerpted from the second volume.

Liber Pontificalis, Life 94, section 40

Meanwhile the blessed pope [Stephen II], ever reflecting on the things of God, had the nighttime offices, which had become slack for a long time, carried out in the hours of the night, and in the same way he restored the daytime office as it had been of old. To the three monasteries which since ancient times perform this office at St. Peter's he added a fourth, and there he established monks who might thenceforth join together in the office, and he ordained an abbot over them. There he bestowed many gifts, both everything necessary for the monks in the monastery, and real estate outside; he established even to this day that with the other three monasteries they should chant in St. Peter's, prince of the apostles.
[Liber Pontificalis, trans. Raymond Davis, p. 68-69]

Further Reading

Raymond Davis, trans., The Book of Pontiffs (Liber Pontificalis to AD 715), Translated Texts for Historians, vol. 6 (Liverpool, 1989).

Raymond Davis, trans., The Lives of the Eighth-Century Popes (Liber Pontificalis), Translated Texts for Historians, vol. 13 (Liverpool, 1992).

Raymond Davis, trans., The Lives of the Ninth-Century Popes (Liber Pontificalis), Translated Texts for Historians, vol. 20 (Liverpool, 1995).