9 There are, indeed, some to whom it has seemed right (and their view is not unreasonable), that it is lawful for the body and blood of the Lord to be offered and received after other food has been partaken of, on one fixed day of the year, the day on which the Lord instituted the Supper, in order to give special solemnity to the service on that anniversary. I think that, in this case, it would be more seemly to have it celebrated at such an hour as would leave it in the power of any who have fasted to attend the service before the repast which is customary at the ninth hour. Wherefore we neither compel nor do we dare to forbid any one to break his fast before the Lord's Supper on that day. I believe, however, that the real ground upon which this custom rests is, that many, nay, almost all, are accustomed in most places to use the bath on that day. And because some continue to fast, it is offered in the morning, for those who take food, because they cannot bear fasting and the use of the bath at the same time; and in the evening, for those who have fasted all day.
10. If you ask me whence originated the custom of using the bath on that day, nothing occurs to me, when I think of it, as more likely than that it was to avoid the offense to decency which must have been given at the baptismal font, if the bodies of those to whom that rite was to be administered were not washed on some preceding day from the uncleanness consequent upon their strict abstinence from ablutions during Lent; and that this particular day was chosen for the purpose because of its being the anniversary of the institution of the Supper. And this being granted to those who were about to receive baptism, many others desired to join them in the luxury of a bath, and in relaxation of their fast.
Having discussed these questions to the best of my ability, I exhort you to observe, in so far as you may be able, what I have laid down, as becomes a wise and peace-loving son of the Church. The remainder of your questions I purpose, if the Lord will, to answer at another time.
Source: Letters (New Advent)