John Cassian, The Institutes III.11: Of the points in which the service held on Sunday differs from what is customary on other days

Of the points in which the service held on Sunday differs from what is customary on other days.

But we ought to know this, too, that on Sunday only one office is celebrated before dinner, at which, out of regard for the actual service and the Lord’s communion, they use a more solemn and a longer service of Psalms and prayers and lessons, and so consider that Tierce and Sext are included in it. And hence it results that, owing to the addition of the lessons, there is no diminution of the amount of their devotions, and yet some difference is made, and an indulgence over other times seems to be granted to the brethren out of reverence for the Lord’s resurrection; and this seems to lighten the observance all through the week, and, by reason of the difference which is interposed, it makes the day to be looked forward to more solemnly as a festival, and owing to the anticipation of it the fasts of the coming week are less felt. For any weariness is always borne with greater equanimity, and labour undertaken without aversion, if some variety is interposed or change of work succeeds.