Every expansion comes with a pre-patch - the real X.0 patch that turns on the system updates of the expansion, but does not let us access the new continent, new levels, new races, or new classes*.
*Except that this time, those who have ordered the expansion will actually be able to create their Demon Hunters as soon as the patch goes live. You'll still have to buy the expansion if you want to play one of these Illidari disciples, but you can go through the starting experience and run some dungeons or raids to get a feel for how the class plays and be ready to hit the Broken Isles immediately with all the other players who are playing existing classes. Remember how it was a while in Mists of Pandaria before you started seeing Monks around (it was also a bit of a delay to see Death Knights in Northrend, though far less of one, given that they only had to make up fifteen levels, not 84.)
A supposed internal leak has identified July 19th as the launch of that pre-patch, which is a Tuesday, making this pretty plausible. The patch would also come out a little more than a month before the expansion's true launch. This is also within the realm of plausibility, especially if the pre-launch events (which always come in these pre-patches) are really as big as they claim they'll be. The biggest expansion launch had to be Cataclysm, which was actually two patches - one to implement the new systems and the Elemental Invasion, and the other to implement the actual Shattering, which allowed new race/class combos and the reworked zones with new quests. The latter happened two weeks before the expansion's launch, whereas the former lasted I want to say either three or four weeks.
So we might be dual-wielding glaives less than a week after Bastille Day.