I personally don't get why they still think the time of year is important. Sure, going up against competition is going to affect things, but why is it all summer or winter? Who cares if the movie comes out February 1st rather than June 1st? People don't only go to the movies two days a year or something for crying out loud.
If anything, like most business models these days, it all seems self perpetuating. They only release movies at specific times. Those movies do well. So, clearly, it was the timing of their release that was the crux of the success.