“But let not your fasts be with the hypocrites (Matthew 6:16); for they fast on the second [Monday] and fifth day [Thursday] of the week; but fast on the fourth day [Wednesday] and the Preparation [Friday]” (Didache 8:1).
First, let’s not get legalistic about this. It got so bad in the fourth century that if a layperson did not fast during these times, unless he was sick, he was to be excommunicated (Canon 69 of The Apostolic Canons). Fasting was created for man, not man for fasting.
But fasting is important, besides the physical benefits. It is a strong spiritual medicine that can heal many afflictions (Mark 9:29). If Jesus’ needed to fast, how much more do we (Matt. 4:2). The early believers fasted (Acts 13:2; 1 Cor. 7:5). Therefore, we should fast.
The easiest way to incorporate fasting into your routine is to follow the practice of the early church and fast every Wednesday and Friday. And how do we do this? St. Epiphanios says: “We fast on Wednesday and Friday until the ninth hour.” St. Benedict (Canon 41) also designates that the fast of Wednesday and Friday is until the ninth hour, which is 3 pm.
And why did the church choose Wednesday and Friday? According to the Holy Hieromartyr Peter (Canon 15): “On Wednesday because on this day the council of the Jews was gathered to betray our Lord; on Friday because on this day He suffered death for our salvation.”
Source
Exomologetarion by St. Nicodemus the Hagiorite