Why TV Show Seasons Begin in September

I had always assumed that September's back-to-school timing was the reason that television shows almost always began their new seasons at that time.  But in the early days of TV, the only major national advertisers were automobile companies.  And the car companies rolled out their new models in September, so that is when the big advertising dollars were up for grabs.  This TV-advertising model has been the standard ever since.

But now things are changing and viewers are demanding new content during the summer and at other non-traditional times.  The television executives are finally moving past the 1950's...