27 As to the seventh day of the week there is less difficulty in acting on the rule above quoted, because both the Roman Church and some other churches, though few, near to it or remote from it, observe a fast on that day; but to fast on the Lord's day is a great offense, especially since the rise of that detestable heresy of the Manichæans, so manifestly and grievously contradicting the Catholic faith and the divine Scriptures: for the Manichæans have prescribed to their followers the obligation of fasting upon that day; whence it has resulted that the fast upon the Lord's day is regarded with the greater abhorrence. Unless, perchance, some one be able to continue an unbroken fast for more than a week, so as to approach as nearly as may be to the fast of forty days, as we have known some do; and we have even been assured by brethren most worthy of credit, that one person did attain to the full period of forty days. For as, in the time of the Old Testament fathers, Moses and Elijah did not do anything against liberty of eating on the seventh day of the week, when they fasted forty days; so the man who has been able to go beyond seven days in fasting has not chosen the Lord's day as a day of fasting, but has only come upon it in course among the days for which, so far as he might be able, he had vowed to prolong his fast. If, however, a continuous fast is to be concluded within a week, there is no day upon which it may more suitably be concluded than the Lord's day; but if the body is not refreshed until more than a week has elapsed, the Lord's day is not in that case selected as a day of fasting, but is found occurring within the number of days for which it had seemed good to the person to make a vow.
28. Be not moved by that which the Priscillianists (a sect very like the Manichæans) are wont to quote as an argument from the Acts of the Apostles, concerning what was done by the Apostle Paul in Troas. The passage is as follows: “Upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.” Afterwards, when he had come down from the supper chamber where they had been gathered together, that he might restore the young man who, overpowered with sleep, had fallen from the window and was taken up dead, the Scripture states further concerning the apostle: “When he therefore had come up again, and had broken bread, and eaten and talked a long while, even till break of day, so he departed.” Far be it from us to accept this as affirming that the apostles were accustomed to fast habitually on the Lord's day. For the day now known as the Lord's day was then called the first day of the week, as is more plainly seen in the Gospels; for the day of the Lord's resurrection is called by Matthew μία σαββάτων, and by the other three evangelists ἡ μία (τῶν) σαββάτων, and it is well ascertained that the same is the day which is now called the Lord's day. Either, therefore, it was after the close of the seventh day that they had assembled—namely, in the beginning of the night which followed, and which belonged to the Lord's day, or the first day of the week—and in this case the apostle, before proceeding to break bread with them, as is done in the sacrament of the body of Christ, continued his discourse until midnight, and also, after celebrating the sacrament, continued still speaking again to those who were assembled, being much pressed for time in order that he might set out at dawn upon the Lord's day; or if it was on the first day of the week, at an hour before sunset on the Lord's day, that they had assembled, the words of the text, “Paul preached unto them, ready to depart on the morrow,” themselves expressly state the reason for his prolonging his discourse—namely, that he was about to leave them, and wished to give them ample instruction. The passage does not therefore prove that they habitually fasted on the Lord's day, but only that it did not seem meet to the apostle to interrupt, for the sake of taking refreshment, an important discourse, which was listened to with the ardour of most lively interest by persons whom he was about to leave, and whom, on account of his many other journeyings, he visited but seldom, and perhaps on no other occasion than this, especially because, as subsequent events prove, he was then leaving them without expectation of seeing them again in this life. Nay, by this instance, it is rather proved that such fasting on the Lord's day was not customary, because the writer of the history, in order to prevent this being thought, has taken care to state the reason why the discourse was so prolonged, that we might know that in an emergency dinner is not to stand in the way of more important work. But indeed the example of these most eager listeners goes further; for by them all bodily refreshment, not dinner only, but supper also, was disregarded when thirsting vehemently, not for water, but for the word of truth; and considering that the fountain was about to be removed from them, they drank in with unabated desire whatever flowed from the apostle's lips.
29. In that age, however, although fasting upon the Lord's day was not usually practised, it was not so great an offense to the Church when, in any similar emergency to that in which Paul was at Troas, men did not attend to the refreshment of the body throughout the whole of the Lord's day until midnight, or even until the dawn of the following morning. But now, since heretics, and especially these most impious Manichæans, have begun not to observe an occasional fast upon the Lord's day, when constrained by circumstances, but to prescribe such fasting as a duty binding by sacred and solemn institution, and this practice of theirs has become well known to Christian communities; even were such an emergency arising as that which the apostle experienced, I verily think that what he then did should not now be done, lest the harm done by the offense given should be greater than the good received from the words spoken. Whatever necessity may arise, or good reason, compelling a Christian to fast on the Lord's day—as we find, e.g., in the Acts of the Apostles, that in peril of shipwreck they fasted on board of the ship in which the apostle was for fourteen days successively, within which the Lord's day came round twice, — we ought to have no hesitation in believing that the Lord's day is not to be placed among the days of voluntary fasting, except in the case of one vowing to fast continuously for a period longer than a week.
Source: Letters (New Advent)