In The Story of Christianity,* author Justo Gonzalez offers the following:
There were indeed times set aside for sorrow for one's sins, in particular during the two weekly days of fasting, which the church adopted from Jewish practice. At an early date, however, at least some Christians began fasting, not on Mondays and Thursday, like the Jews, but rather on Wednesdays and Fridays. It may be that this shift took place in commemoration of the betrayal and the crucifixion.
Was this fasting practiced in a majority of early churches? If so, at what point in history did the tradition cease? Is there more information as to why the days of Wednesday and Friday were chosen over Monday and Thursday?
* Revised edition, Volume 1, Chapter 3.