Avatar 2 finally looked like it was going to hit theaters in 2018, but in March 2017, Cameron revealed that they weren't going to be able to make that release date. A month later, Fox announced the first sequel would release on December 18, 2020, with the three other sequels releasing every December after that. It was surprising considering that Lucasfilm has been pushing out their Star Wars movies on that same weekend for the past few years, and it seemed like they would be doing so for the foreseeable future.
However, a few days later, Disney came out their own announcement. As it turns out, the House of Mouse had decided to move Star Wars: Episode IX back to its prime May slot, thereby freeing up December for Avatar. Perhaps the two studios reached some sort of mutually beneficial agreement. After all, it wouldn't be the first time Disney cooperated with another studio. In 2015, Disney agreed not to promote Gareth Edwards's Rogue One: A Star Wars Story outside of conventions until after Christopher McQuarrie's Mission: Impossible—Rogue Nation released in theaters, so as to prevent public confusion between the two films.
On the surface, it may seem like a coincidence, but the fact is, Cameron has stated multiple times that he wouldn't want to compete with Star Wars. No one would. As he told Collider in April 2016, he doesn't "want to land on the same date as one of the Star Wars sequels" because "that's just good business." Cameron then further elaborated, saying, "I don't want to go head-to-head with Star Wars. That would be stupid. And hopefully, they won't want to go head-to-head with [one of the Avatar sequels]."
But if Star Wars and Avatar did go to war, man, that would be the box-office battle of the ages.