Fasting means going on a disciplined diet. And like all real diets it should be hard.
The purpose is to find out who is in charge: You or your stomach. The battle between the two tempers the will, heightens spiritual sensitivity, and leads us to find strength in God through prayer.
The first Christians fasted as a spiritual discipline and as a way of accompanying our Lord in his suffering. They fasted on Wednesday because Judas conspired to betray Christ on a Wednesday; they fasted on a Friday since that is the day of Christ’s crucifixion and death.
Their fast consisted mostly in avoiding meat since meat was mostly for the wealthy. Fish and vegetables were much more easily obtained. This is why the tradition of the most common form of fasting (or abstinence if you prefer) is to omit meat and eat fish.
Fasting has taken a hit in our western culture; we give up meat, but we enjoy a sea food platter in its place. Perhaps this is because they’ve missed the point on fasting: It should hurt, as it is a means of participating in the Lord’s suffering.