When it comes to Persona 5 releasing in Japan, I really feel we'll get the release date announced by March.
I can't imagine them being overly silent anymore like they were in the past. They've shared too many details now for that to be the case, in my opinion. I'm expecting radio silence in Q4 2015, but not in Q1 2016. They're going to have to give us the release date as the Spring season begins, or else people will become concerned. Hashino himself has pretty much stated that the game is complete and that they're polishing it up and making sure the game works well with the scope they've wanted. So, to not show anything by March would look very bad. I can't imagine the release date being announced in May, as that's too far into the season, and cuts close to when Summer begins. I don't think ATLUS would make fans wait that long for the release date. There's no other game coming out by ATLUS in 2016 besides this and Odin Sphere: Leifthrasir, which releases on January 14, 2016.
I'm expecting the game to launch in July 2016, as it feels the most likely judging by past Persona mainline releases by the P-Studio, and the fact that for the most part, when ATLUS JP says "Summer" it's usually July, with P4U2 being the exception.
Sorry for the rant. I just decided to post this because I saw people discussing P5, and wanted to share my opinion.
As for when we'll see a full English trailer: I'm honestly expecting E3 2016, because I've noticed that's when Atlus USA does that for the most part.