Hump Day is simply the middle day of a work week. In many places the work week is from Monday to Friday, making Wednesday Hump Day.

A hump is a raised mound of earth. It can prevent you from seeing something else. Once you make it past Hump Day, you are now closer to the next weekend and can see it. The rest of the week at work seems to go by quicker and is not as painful or boring once you have passed Hump Day.