Well I think SW probably did start the trend of big special effects summer blockbusters like we're so used to seeing now. Since I don't think that was nearly as common beforehand.
i'm afraid that even the "big summer blockbuster" claim is vastly overstated. Summer has always been a big time for films. Children are out of school, and an air conditioned movie theater is a good way to get out of the hot weather. So while I can't say for sure what the first big summer movie was, I can say for sure it wasn't Star Wars.
Jaws: released July 20, 1975
Return of the Pink Panther: May 21, 1975
Cannonball: July 6, 1976
The Omen: June 25, 1976
Smokey and the Bandit: May 19, 1977
Smokey And The Bandit was the no. 2 movie at the box office in 1977, Star Wars was no. 1 by a wie margin. And none of those movies made the 400 million plus that Star Wars did. The case can be made for Jaws being the first blockbuster and possibly CE3K but none of the others really came close to Star Wars.