Okay, the biggest question in the fandom besides 'when will Avatar air?' is 'why isn't Avatar airing?'
I think I have an answer.
Watch the first 2 minutes of this video and ignore the dreadful picture quality.
Basically, Bryke say in this video that they're about 2 weeks away from finishing the finale. Which means the finale isn't finished yet (well it is by now, but my point still stands).
Maybe Nick hasn't been airing new episodes because they want to wait until the finale is completed.
Maybe they released the book and DVD early because long ago they thought that the finale would've been aired by now (which it couldn't have been anyway), so they set a release date for the book and DVD (which I hear is notoriously hard to change once it's been set).
Maybe Nick isn't as much to blame as we think they are. Maybe Bryke are to 'blame' for taking a long time to make the finale to make it as epic as possible.
And for die-hard Nick haters, remember they're still partially to blame for ordering season 3 late.
So, yeah, they can't air the finale now, because it's only just been finished.
And maybe they haven't been airing the episodes before that (312-317) because they want to air the last 10 episodes closely to each other.
Of course that still raises the question of why they're not airing it in July.
Maybe they didn't know when it would be finished, so they played it safe by making it in July, just before the predetermined DVD release date.
So, yeah, don't just mindlessley bash Nick and don't just mindlessley worship Bryke, both are only human.
/Tomtitan ranting. Flame me please for not agreeing with you.