Journey to Un'Goro will release on April 6, barring any unforeseen circumstances of course.
Yesterday we got our first extended glimpse at Hearthstone's Journey to Un'Goro expansion pack, and now it looks like we have a solid release date. The official timeline Blizzard gave us was "mid-April", but thanks to some shenanigans with the in-game client, and an Amazon coins promotion, we are now fairly certain that the exact release date is April 6.
The folks over at Hearthpwn first discovered the release date by utilizing an old trick with the game's client. By setting their PC's clock forward day-by-day, they could check which day the pre-ordered Journey to Un'Goro cards become openeable. This has been a common tactic to figure out release dates of content in the past and has an excellent track record of being correct.
If that isn't enough proof for you, Amazon is currently running an Amazon coins promotion, where customers can get 500 bonus coins if they pre-order Journey to Un'Goro on Amazon. In the terms and conditions, it states that the promotion will end at 11:59 PM on April 5.
So there you have it, barring any extrenous circumstances the expansion should launch on April 6. To check out all of the cards revealed so far, click here.