Update #3: Blizzard has confirmed that PvP Season 3 will conclude on July 19th. This makes it very likely that the pre-patch will arrive on July 19th.
Update #2: A news post has been published on the Battle.net Blog. It states the following:
Update #1: We have contacted Blizzard which has declined to comment on the pre-patch release date or pre-load.
Original Story: Before World of Warcraft: Legion launches on August 30th, several things will happen. For one, an invasion will strike Azeroth, introducing highly anticipated new content that preludes the expansion. Around this time those who have pre-ordered the expansion will gain access to a new class: the Demon Hunter. But before any of these things will happen, a Legion pre-patch will become available.
Many World of Warcraft players—including myself—are extremely excited about the Legion pre-patch. Not only will it deliver new UI features and an account-wide transmogrification menu, but it has big changes in-store for every class spec. Naturally, many players have been discussing when they expect the pre-patch to debut, hoping that it arrives sooner rather than later.
I am happy to report that the Legion pre-patch is currently available for pre-load on the live servers. I'm currently downloading it alongside thousands of other players. It weighs in at nearly 20 gigabytes, making it one of the largest patches in World of Warcraft history.
If you boot up Battle.net and don't immediately begin downloading the patch, try heading into the Settings menu just right of the Play button. From there, click "Check For Update" to retrieve the patch.
Blizzard is yet to announce when the pre-patch will hit the live environment. What we do know is that patches always debut on Tuesdays during maintenance. We also know that Blizzard gives two-week warnings for the end of each PvP season. Since this update will conclude a PvP season, it is anticipated that Blizzard will give a two-week heads up in anticipation of the patch.
At this point, the rumored date of July 19th seems very probable. Stay tuned for the final word from Blizzard.