30 The reason why the Church prefers to appoint the fourth and sixth days of the week for fasting, is found by considering the gospel narrative. There we find that on the fourth day of the week the Jews took counsel to put the Lord to death. One day having intervened—on the evening of which, at the close, namely, of the day which we call the fifth day of the week, the Lord ate the passover with His disciples—He was thereafter betrayed on the night which belonged to the sixth day of the week, the day (as is everywhere known) of His passion. This day, beginning with the evening, was the first day of unleavened bread. The evangelist Matthew, however, says that the fifth day of the week was the first of unleavened bread, because in the evening following it the paschal supper was to be observed, at which they began to eat the unleavened bread, and the lamb offered in sacrifice. From which it is inferred that it was upon the fourth day of the week that the Lord said, “You know that after two days is the feast of the passover, and the Son of man is betrayed to be crucified;” and for this reason that day has been regarded as one suitable for fasting, because, as the evangelist immediately adds: “Then assembled together the chief priests and the scribes and the elders of the people unto the palace of the high priest, who is called Caiaphas, and consulted that they might take Jesus by subtlety and kill Him.” After the intermission of one day—the day, namely, of which the evangelist writes: Now, on the first day of the feast of unleavened bread, the disciples came to Jesus, saying unto Him, Where will You that we prepare for You to eat the passover? — the Lord suffered on the sixth day of the week, as is admitted by all: wherefore the sixth day also is rightly reckoned a day for fasting, as fasting is symbolic of humiliation; whence it is said, “I humbled my soul with fasting.”
31. The next day is the Jewish Sabbath, on which day Christ's body rested in the grave, as in the original fashioning of the world God rested on that day from all His works. Hence originated that variety in the robe of His bride which we are now considering: some, especially the Eastern communities, preferring to take food on that day, that their action might be emblematic of the divine rest; others, namely the Church of Rome, and some churches in the West, preferring to fast on that day because of the humiliation of the Lord in death. Once in the year, namely at Easter, all Christians observe the seventh day of the week by fasting, in memory of the mourning with which the disciples, as men bereaved, lamented the death of the Lord (and this is done with the utmost devoutness by those who take food on the seventh day throughout the rest of the year); thus providing a symbolic representation of both events—of the disciples' sorrow on one seventh day in the year, and of the blessing of repose on all the others. There are two things which make the happiness of the just and the end of all their misery to be confidently expected, viz. death and the resurrection of the dead. In death is that rest of which the prophet speaks: “Come, my people, enter into your chambers, and shut your doors about you: hide yourself as it were for a little moment, until the indignation be overpast.” In resurrection blessedness is consummated in the whole man, both body and soul. Hence it came to be thought that both of these things [death and resurrection] should be symbolized, not by the hardship of fasting, but rather by the cheerfulness of refreshment with food, excepting only the Easter Saturday, on which, as I have said, it had been resolved to commemorate by a more protracted fast the mourning of the disciples, as one of the events to be had in remembrance.
Source: Letters (New Advent)