The Hearthstone team has designated Saturday May 20th as Free Card Day. As the name suggests, you get a free card for participating in this event, however you have to do surprisingly little to actually qualify for the reward!
Simply log in to Hearthstone at some point tomorrow, and Fight Promoter will automatically be added to your collection. You're at leisure to use this card in your decks, or simply send it to the dust mill if you'd rather put it to use elsewhere.
There are no strings attached to this event whatsoever, so simply ensure you log in at some point tomorrow and claim your freebie.