Blizzard has made a new post on their website detailing the upcoming Legion pre-patch.
EDIT 3: Up top because it's the most indicative of a release. The current PvP season is ending on July 19th. While Blizzard specifies that this does not require a patch, that strongly suggests that the patch will be the 19th. It is possible we'll see the 26th, especially because of this note about not needing a patch, but I think the 19th is more likely. See the bottom of the post for updates about the Grove Warden and when Demon Hunters will become playable.
Specifically, it lists the following features:
I know I've said this for every expansion, but classes are changing more than they ever have this time around, with every spec getting stripped down to its core and often built back up with new mechanics. Survival went from being a ranged spec to a melee spec. Combat got renamed to Outlaw and now has a much stronger "pirate" theme. But even specs getting less visible changes are still changing quite a bit.
One of the two game-changing features of 4.3, transmogrification is now far more alt- and bag-space-friendly in 7.0. A new tab has been added to the collections window called Appearances. Any time you get a new piece of soulbound gear (if it's BoE you'll have to equip it) that is appropriate to your class (so Hunters can't collect Cloth gear appearances,) you will forever have the look of that gear stored in your collection for any character on the account to transmog it (with racial and class restrictions accounted for.)
Burning Legion Invasion:
Several zones will be under assault from the Burning Legion, where players will have to fight off hordes of demons, receiving item level 700 gear, including weapons that can be further upgraded with a currency that can also be used to purchase gear and a new battle pet. I actually think there's other stuff to this too in the major cities as well, but I haven't seen it on the PTR yet.
Broken Shore Event:
There is a massive scenario sending you to the Broken Shore to make the first attempt of an assault on the Legion's base camp, which is followed by a quest line through your capital and gives you a taste of plot to come.
Demon Hunters Early Access:
In the coming weeks, players who have pre-purchased Legion will be able to start their Demon Hunter characters before the expansion proper launches. Demon Hunters begin at level 98 and will hit 100 in the course of their starting experience so that players who wish to take their Illidari heroes into the fray first can do so along with all the veteran characters.
It seems clear from the announcement that Demon Hunters will not be available the moment the patch hits, but likely only after a week or two of invasion. When exactly they will arrive is anyone's guess.
So When Is It?
That's really the question. Previously I suggested we'd be seeing it August 2nd or July 26th, but this announcement suggests to me that they might be getting ready for a launch soon. They could launch on the 12th, though that might be a little short-notice (for one thing they need to announce the end of the current PvP season ahead of time.) So I think the 19th is definitely within the realm of possibility.
Then, of course, there's Demon Hunters. Given that we don't know when the patch will hit, I think it's a stronger base of speculation to go by the expansion's release date and work backward. Considering that Cataclysm gave us two weeks with the new race-class combinations, I think the 16th of August is a solid possibility. That said, given that the Illidari play a big role in the Legion invasions and the city quest lines, it's also possible they'll try to get them available early (that said, these quest lines definitely treat the Illidari as new outsiders, and REDACTED sends you as an ambassador to them, which would be odd for someone who's already a member.
Anyway, get excited. And if you, like me, have not yet gotten your Grove Warden mount, now's the time to decide if you're still going to try. (The mount is gorgeous, but I don't think I have the patience to find/pay a group to take me.)
EDIT: A blue post just clarified that the Grove Warden will actually be available until the expansion itself launches, so you'll technically be able to get it on your Demon Hunter if you gear up quickly.
EDIT 2: Looking closer, the post specifies that Demon Hunters will be available no later than August 17th, which is two weeks before the expansion launch. They are giving themselves the opportunity to do it earlier, but for now I'd count this as the official launch for Demon Hunters to become playable (though maybe the 16th is more likely, as the 17th is a Wednesday and big patch-like things tend to be on Tuesdays.) But the key is, if you've pre-ordered, you will definitely be able to play your Demon Hunter in the two weeks leading up to the expansion's launch - plenty of time, I think, to get them caught up gear-wise, as I believe they start with 680 gear and the invasions give 700 gear.