Aquaman's not going to be ready for the Summer. That's why they pushed it back from July in the first place. Plus, if shit goes sideways with JL, Wan will want as much time as possible to get everything right. Not to mention this movie is going to be super CG heavy.
Judging from the reactions to him in the little time we've seen him in the JL trailers, that move's gonna happen.
The first weekend in December is clear of anything. It would be a few weeks removed from Wreck It Ralph 2 and a week before the animated Spider-Man movie. Much easier spot to be in than dealing with both Spider-Man and Aquaman opening weekend.