Based upon the 1973 play, it was released in 1975, and failed.
Then on April 1st, 1976, it was played at the Waverly theatre at midnight. It was a success, and rapidly developed a large following. The regulars started dressing up as characters from the movie and even started yelling things back at the screen. This escalated to the audience participation we have today.
Many now-famous people were in the cast.
Shock Treatment, the non-sequel came out in 1983.