Blizzard is set to ramp-up the events in Azeroth leading up to the release of the World of Warcraft Legion expansion which will include the new Demon Hunter.
In a live community stream today focusing on the new class, they have revealed that the Demon Hunter will be available to play from 9 August as part of the Legion pre-expansion updates. They will go live with maintenance on 9 August and early 10 August in Europe.
The Demon Hunter will launch alongside the Demon Invasions event. You can watch it all in this video.