As many have discovered, this week started a large 20+Gb background download of the Legion pre-patch. The World of Warcraft community site has been updated with a bit more information about what players can expect as far as what systems will go live during the pre-Legion event and, most importantly, when the new Demon Hunter class will be ready for play!
During the pre-launch update, players will be able to take advantage of the new transmog system that allows them to see all collected gear and what can be used to change the appearance of equipped items. In addition, the numerous and all-encompassing class specification changes will be implemented as well.
For Legion pre-launch adventures, players will be able to take part in worldwide Burning Legion invasions, be able to get started playing the new Demon Hunter class and will take part in the battle of the Broken Shore. Demon Hunters will enter the game no later than August 17th.
Lastly, the current PvP season will end on July 19th, usually given two weeks before a raid or expansion pre-event leak. It's possible that the pre-event will begin as early as August 3rd.
Legion is expected to launch on August 30th.
Read more details on the World of Warcraft site.