Heheh, in fact ... in November and December of 1978, there was a multiplex on Long Island playing both Star Wars and Close Encounters and I'd sit through a (stolen) double feature of both, like 3 or 4 nights a week. It was 18-year-old fanboy heaven.
It was near the end of the runs for both those films. Star Wars had been in release for a year-and-a-half, and Close Encounters for over a year. Try to picture that happening with any film nowadays. Of course, there was no DVD release coming up, or even a VHS one. But all films did not remain in theaters for over a year. Close Encounters was a huge hit movie, and it was seen in its original form by millions of people.