The Hearthstone team at Blizzard have spoken in the past about wanting to give away more and more free cards to players, and this weekend they're making good on that. Anyone that logs into the game on Saturday May 20 will receive a free Fight Promoter card – a 6 mana, 4/4 card with a Battlecry: "If you control a minion with 6 or more health, draw cards."
Hearthstone director Ben Brode has said on Reddit that this isn't part of a larger test initiative, noting that "we just think it's fun to do surprising things." He went on to note that the idea was somewhat inspired by Free Comic Book Day a promotion that comic book stores ran on May 6, and that "keeping Hearthstone spicy is important...giving out a free card seemed like something easy we could do as a fun one-off. We have some bigger (and smaller) events planned...we're just getting warmed up!"
When asked if Hearthstone would move to a system of free daily login rewards like other free-to-play games – particularly on mobile – Brode noted, "we did it leading up to Un'Goro launch and it was pretty fun. In general, putting aside the question of 'how much is the right amount of free stuff?' the question is 'what's the right way to deliver the free stuff?' For example, we added ranked rewards, better quests, tavern brawl packs, etc. I'm not against finding other ways to reward stuff, but is daily log-in the best way? I tend to find it tedious to have to remember to log in every day. It makes me sad when I miss a day...I do think it's cool to have a fun present to open every day, though. There are pros and cons to a system like that."