I believe Lucasfilm wants to move Star Wars to the May period anyways and away from December (even though it's working for them) starting with the Han Solo movie. FOX will likely retain December for Avatar once they start rolling out. That's what I think will happen. But even if they both try to go for December, I guarantee one of them will blink. FOX and Disney will both not allow this head to head to occur.
Nah, it has to be all-out war for financial dominance! Damn the consequences, one shall stand, one shall fall! It's like when people honestly thought Civil War and Batman v Superman were going to open on the same day. Like, they seriously entertained this notion.
As it is, the way modern box-office works, you only really need about 3 weekends tops without equivalent competition. Unless you're a juggernaut of unforeseen proportions. In which case it won't matter at all.
But a lot of massive films make most of that massive money in their first three-to-four weeks. After that it's just the slow business of legging it out. If Avatar wanted to get some of that holiday money, it could claim Thanksgiving, and that'd give it a month before Star Wars dropped. Or vice versa. It's not as all-or-nothing as everyone wants to make the shit. These studios don't really want to get in each other's way. They just wanna maximize their profits if they can. And increasingly, more studio heads are exploring new ideas on how to do that via their scheduling. And audiences seem to be liking that.
Because the concept of Box Office Seasons was always kinda dumb, honestly. Related to that: The idea that "competition" is a guaranteed bat to the kneecaps for a film doesn't make a whole ton of sense, either. I believe there are studies that state multiple successful films in release at roughly the same time doesn't hurt either film, and in fact, helps the box-office overall, as more people wind up spending time at the movie theater than they normally would.
(also worth noting: Almost every Avatar discussion
immediately becomes a box-office thread, and the large majority of discussion about the film, and its creator, is focused through that specific prism)