If you guys are wondering why Disney have moved Ep 8 from their previously announced December 2017 release date, it's because they aren't stupid enough to put it up against Avatar 2.
It occurred to me, since I have no idea how these deals work, do you think Disney hopes that Avatar 2 is a huge success? It's not their movie, but given their new Avatar Land park, wouldn't Cameron's sequels being successful also be in Disney's interest?