I doubt it, but if you think about it, blockbusters *are* released in December (as well as summer). There was one recently called 'Titanic' if I recall.
Big movies are always (usually) slated to come out around Memorial Day weekend, the 4th of July weekend, and Thanksgiving/Christmas.
And I really don't think Star Wars had anything to do with it. It's widely held that 'Jaws' was the first of what is now commonly called a 'Blockbuster' movie.