probably going to be something with other common cards that haven't been used yet - like Blurr, Trailcutter (what happened to Trailblazer?) HOist, Warpath etc :(
Maybe it'll be mini-cassettes.
They posted something over on their Facebook Page. Although it doesn't have the start date in the description, the event invite says Wedesday (7/24/13):
From: https://www.facebook.com/events/535072859893739/?ref=22
Hound! Aw yeah he is like my favorite autobot!
This is from the events page here on Wiki. Not sure where they will have got their info, but it does quote the same link provided by Ricky. I'd be surprised if an event doesn't start Weds 24th at 5pm PST (Thurs 25th at 1am GMT) anyway.