http://variety.com/2016/film/news/james-cameron-avatar-sequels-toruk-cirque-du-soleil-1201853970/
variety article where he talks about the sequels being a family saga, and he also says the December 2018 is not locked in stone. Not that they aren't going to try for it, but if they miss it, he's fine with that.
Here's his quote from the article:
As for the movie sequels: They’re on target to start rolling out in Christmas 2018 — at least for now. “We haven’t moved that target yet, but we will if we need to,” he said. “The important thing for me is not when the first one comes out but the cadence of the release pattern. I want them to be released as close together as possible. If it’s an annual appointment to show up at Christmas, I want to make sure that we’re able to fulfill on that promise.”