Earlier today in the official patch notes for the 15.6 Patch, Hearthstone announced a Warcraft Anniversary celebration event. We've got the details!

Quote From Patch Notes

To celebrate the 25th anniversary of the Warcraft franchise, players who log into Hearthstone any time between November 13 10:00 AM PT and November 27 10:00 AM PT will receive awesome Warcraft themed rewards. Stay tuned for more details soon!

Login Rewards - Free Cards

Logging in during the anniversary event will reward players with two cards.

New Tavern Brawl Chalkboards

Three new chalkboards were added with the patch, perhaps one of them is a new brawl during the two-week-long "event"?