Movies are released on Friday because it is the weekend after a long work week people want entertainment. Friday is also a Pay Day, for most and is looking for something to do that night. Friday or Saturday is known as also "date nights". Many couples, lover prefer to watch the movie in that night. So the movie maker wants to get maximizing profits from the theaters. The simplest reason one can think is a weekend start with Friday evening and Saturday and Sunday being off.
Suppose a movie being released on a Monday. A good movie would sale from side to side however an awful movie would pack up by Friday without even making any gains. And also the government and the commercial world would be busy clearance the backlog important meetings and the response to the newly released movie would be very low including students, employers and employees etc.
But Movies now release on Thursday Late Night Show at 12am because college students in U.S. are likely to go watch movies on Thursday rather than on a Friday because Friday they go out clubbing and Saturday they party. Thursday is also famous for shopping so that the some people prefer to watch movies on Thursday. The first Hollywood movie "In Old California" was released on Thursday March 10, 1910. So there is not any particular day for releasing the movie. Friday is a common day for releases that's made fully enjoyed to our weekend.