How it was brought about that they fast on the Sabbath in the city.
But some people in some countries of the West, and especially in the city, 752 not knowing the reason of this indulgence, think that a dispensation from fasting ought certainly not to be allowed on the Sabbath, because they say that on this day the Apostle Peter fasted before his encounter with Simon. 753 But from this it is quite clear that he did this not in accordance with a canonical rule, but rather through the needs of his impending struggle. Since there, too, for the same purpose, Peter seems to have imposed on his disciples not a general but a special fast, which he certainly would not have done if he had known that it was wont to be observed by canonical rule: just as he would surely have been ready to appoint it even on Sunday, if the occasion of his struggle had fallen upon it: but no canonical rule of fasting would have been made general from this, because it was no general observance that led to it, but a matter of necessity, which forced it to be observed on a single occasion.
The Saturday fast was observed at Rome in very early days, being noticed by Tertullian, who seems to suggest that it originated in the prolongation of the Friday fast (on Fasting, c. xiv). But it seems to have been almost peculiar to Rome, and at Milan, in the time of S. Ambrose, the Eastern custom prevailed. See the important letter of Augustine to Casulanus (Ep. xxxvi.), where the whole subject of the difference of usage on this matter is fully discussed. The reason here given by Cassian for the origin of the local Roman custom (viz., that S. Peters traditional encounter with Simon Magus took place on Sunday, and was prepared for by the apostle with a Saturday fast) is also there alluded to by Augustine as being the opinion of very many, though he tells us candidly that most of the Romans thought it false. “Est quidem et hæc opinio plurimorum, quamvis eam perhibeant esse falsam plerique Romani, quod Apostolus Petrus cum Simone Mago die dominico certaturo, propter ipsum magnæ tentationis periculum, pridie cum ejusdem urbis ecclesia jejunaverit, et consecuto tam prospero gloriosoque successu, eundem morem tenuerit, eumque imitatæ sunt nonnullæ Occidentis ecclesiæ.” Cf. also Augustine, Ep. ad Januarium, liv.