Here's what I expect: the NX reveal date will just pop up one day in a press release. There will be no lead-up to it or any sort of indication that "oh, we might hear something about a reveal date around this time" and there will have been no way of predicting or expecting the actual date based on what Nintendo has done in the past.
You can narrow down the potential reveal date quite a bit:
- Nintendo historically never makes huge announcements on Monday or Friday. The NX announcement will be on a Tuesday, Wednesday, or Thursday.
- Nintendo probably isn't going to announce the NX in July. Journalists are going to need to make travel arrangements a month in advance so we would have heard about the event by now. You might as well have announced it at E3 if you were going to be ready in July. There's also the political conventions in the US at this time.
- Nintendo isn't going to announce the NX during the Olympics in August. This is a huge distraction and sucks up a lot of media attention.
- Nintendo's going to want to reveal the system before the Holidays (which are another distraction). So anything after mid-November is out.
- Nintendo's going to have to ship near-final hardware dev kits out at some point and it's likely you'd want third parties to have those kits at least for a full six months before launch. This would also allow Western companies to have the dev kits for a year before most of the big multi-platform heavy hitters come in Fall 2017.
- It would be beneficial to announce the NX before TGS so that Nintendo's Japanese partners can announce games for it then.
That gives me:
August 23-25 (I doubt it's this week because the Olympics end on August 21st)
August 30-Sept 1
September 6 & 7 (Sept 5th is after Labor Day holiday in the US, so that's not a likely reveal date)
September 13 & 14 (TGS starts September 15th).