Manual work, and the offices of the third, sixth and ninth hours.
They keep to their manual work privately in their cells without ceasing, but not so as to neglect their study of the psalms or other parts of Scripture. They meditate on them constantly all day, spreading over the whole day what we are accustomed to observe at definite fixed times.
(ibid. chap.3) But they do mark the third, sixth and ninth hours with three psalms apiece. We know that the prophet Daniel poured forth prayers to God at these same three hours in his chamber with the windows open (Daniel 6.10). There are good reasons why these times of day have been specially set aside for religious observances. For at these hours the promises were fulfilled and the great matters of our salvation were accomplished.
At the third hour the holy Spirit descended upon the apostles as the prophets had foretold, giving them the knowledge of tongues (Acts 2.4).
At the sixth hour the spotless victim, Jesus Christ our Lord, was offered up to the Father, mounting the cross for the salvation of the world to wipe out the sins of the human race. At this same hour, Peter’s vocation to the gentiles was revealed to him as he stood in ecstasy, for he witnessed the gospel vessel coming down from heaven, and the cleansing of all the living creatures in it, as he heard the divine voice saying to him, ‘Rise, Peter, kill and eat’ (Acts 10.9 et seq.). The fact that the vessel was let down by the four corners signifies nothing other than the four Gospels.
At the ninth hour Christ went down to the lower regions and extinguished the impenetrable darkness of Tartarus by his own shining splendour, bursting open the gates of bronze and breaking the iron bars, taking back with him to heaven the captive band of saints, and by the removal of the fiery sword (Genesis 3.24) restoring to paradise its original inhabitant. At this same hour Cornelius the centurion stood in prayer and knew by the message of an Angel that his alms and prayers had been accepted by God (Acts 10.3).
It is clear then from these examples that just as these holy men and apostles devoted these hours to religious observances so should we do likewise. If we had no rule binding us at the very least to some definite times for these devout duties, we would spend all day wrapped up in forgetfulness, idleness and useless pastimes.
From: De Vitis Patrum, Book IV, Chapter 20 (Cassian, Institutes, Bk.3, chap.2)