We know from the Didache (c. AD 60) that fasting on Wednesdays and Fridays is an ancient practice of the Church, hearkening all the way back to the first century AD.

While some have come to think that this was established as an antithesis to Judaic fasting (which was done on Mondays and Thursdays), it is more likely that this was a practice inherited from Second Temple Judaism itself.

“After the discovery of the Qumran documents [...] it may be considered as proved that the origins of this tradition lie in the ancient sacred calendar which the Essenes observed and which in all probability was accepted by the Judeo-Christian communities in Palestine. Wednesday and Friday have special significance in this calendar. Christians appropriated these days and later added a new meaning to them – as commemorations of the days of Christ’s betrayal and His death.”
Fr Alexander Schmemann, Introduction to Liturgical Theology, pp. 156-157