The way I remember it, a show produced 39 episodes a year. The best 12 of those episodes ran again in the summer. They were called summer reruns. So you had a slight chance to see the show you missed, if you weren't busy with a summer job or a date or vacationing or something.
Yeah, that only adds up to 51 episodes a year, but most productions got a week off at Christmas, when the network ran Christmas specials or movies. Modern series only run 12-20 episodes a year these days. (Thanks, WTM!)