User 和歌音 from the National Geographic of Azeroth (NGA) has uncovered compelling evidence that the release date for Whispers of the Old Gods will be April 26 2016.

The NGA is a Chinese World of Warcraft forum, and user cross the information from there to Reddit. It seems that if you try to buy the Curse of Naxxramas adventure, there is a pop-up that warns you that the cards will only be playable in Wild format after April 26.Although we have just had April Fool's, other players have tested this and found the same result. It is very likely that this is the release date for the new set, and so you can now book the surrounding days off work.