Sundays are not Fast-Days

They said to him, 'The disciples of John are always fasting and offering prayers, and so are those of the Pharisees, but yours just eat and drink.' And Jesus said to them, 'You can't make the groom's friends fast as long as the groom is present, can you? But the days will come when the groom is taken away from them, and then they will fast, in those days.'

--Luke 5:33-35, SV

"Even before the introduction of Lent it had been customary to fast before Easter: one day, two days, even a week. But even when Lent was generally accepted, not all of its forty days [from the First Sunday of Lent until Holy Thursday] were at first regarded as fast days. In Rome toward the end of the fourth century a fast of three weeks was usual; and even when people began to fast on all the other days of Lent they still made an exception of the Sundays. Because Lent contains six Sundays, there thus remained thirty-four fast days leading up to the ancient paschal triduum. But if Good Friday and HOly Saturday (were also fast days) were counted as well, that made thirty-six days in all--just one tenth of a year. In this fashion, as was observed with a certain satisfaction (for example, by John Cassian and Gregory the Great), one paid a tithe of the year to God.

"But since the seventh century considerable importance began to be attached to the idea that in Lent there ought to be the full number of forty fast-days. It became necessary, therefore, to take in four days from the preceding week; and thus Ash Wednesday came to be the beginning of Lent."

Josef Jungmann