So, why is Wednesday called "Hump Day"?
The answer is almost obvious. According to the Urban Dictionary, "Hump Day is the middle of a workweek. It is called "Hump Day" in the context of climbing a proverbial hill to get through a tough week."
Wednesday, the day in the middle of the week, implies that you have to get "over the hump" before you can anticipate the weekend.
Wednesday is in the middle of the common Western 5-day workweek that starts on Monday and ends on Friday. Also, when Sunday is taken as the first of every week, Wednesday is the day in the middle.
Some people say Wednesday is the absolute best day of the week. They say it is the day of maximum hope because they have made it through two days of the workweek and have only two more days to work before they have a two-day break.
Hump day symbolizes the height or the peak of the workweek. You have climbed the hill and made it to the hump (or the top). After Wednesday the workweek is supposed to be easier because everything is down hill from that point on.