Some people claims the Disco scene started already in the early 70's. Like in '71-'73. But I guess that really depends on what you think is DISCO?! The Discotheque scene really started back then, with clubs starting playing music on records to the audience. In these clubs, the Disc-Jockey [the DJ] was formed as this guy putting the records on the turntables and often talking to the crowd in a radio show kind of way.
Other people say the Disco days started in the mid 70's and that's also my opinion. I think that you in tunes from around '74-'76 can start hearing more of, what I call, "the Disco sound" - than you could just a couple of years before.
One of the first "Disco Hits" was Gloria Gaynor's" Never can say goodbye", which was on top of the charts already back in 1974.