In the early Church every Thursday was a mandatory day of feasting, not fasting.
In the Catholic Church various minor traditions have come and gone over the years. One tradition that influenced the way Catholics approached fasting (and the Lenten season) was a biblical association with Thursdays.
Traditionally the Catholic Church always celebrated the ascension of Jesus Christ into Heaven on a Thursday (40 days after Easter). This led to a tradition in the early Church of every Thursday honoring Jesus’ ascension with a day of great rejoicing.
Also, Thursday was the day of the Last Supper, the day when Jesus officially instituted the Holy Eucharist. So Thursdays were seen to be full of reasons to feast, and as a result it was a day when fasting was strictly forbidden.
Thursday was also exempt from fasting so as to distinguish Christian practice from Jewish fasting customs. Aside from the major days of fasting, at that time Jews fasted on the Monday and Thursday following Passover. Christians chose Wednesdays and Fridays as days of fasting, as the 1st-century manuscript called the Didache mentions.
Wednesdays were associated in the early Church with the betrayal of Judas (who betrayed Jesus on a Wednesday) and Fridays were always connected to the first “Good Friday” when Jesus died on the cross. Those two days were deemed the most appropriate days for fasting by the early Christians.
This exaltation of Thursdays may have also impacted the early Christians observance of Lent. For example, Lent for some Christians may have started 70 days before Easter. As the Catholic Encyclopedia notes, “it may simply denote the earliest day on which some Christians began the forty days of Lent, excluding Thursday, Saturday, and Sunday from the observance of the fast.”
However, this practice was eventually dropped as feast days for saints became more important in the liturgical calendar and the biblical associations with certain days of the week were no longer emphasized (except Sunday).
While this tradition is no longer observed within the Catholic Church, it reminds us that days of the week can remind us of particular events within salvation history. In a certain sense, we can relive the life of Christ every week, remembering certain events from his life each day.