For the 2015 Holiday Sale
Earlier this year I got a jump on the Steam Summer Sale event by happening to craft a badge at the right time. As luck would have it, it happened again. No, I don’t just always craft badges. Okay maybe I do.
I noticed this evening after crafting the first level of the Undertale badge that I got an emoticon (Flowey) and a background (Sans), but I didn’t get anything else. That’s not entirely true though. Steam didn’t explicitly tell me I received anything else, but my inventory had the card above in it.
As in the past few years, these Mysterious trading cards have signaled the lead-in to the big Steam sales. By collecting the full set of Steam Sale trading cards, you can craft a unique badge. If you start crafting badges now to begin collecting these Mysterious cards, they should convert to the proper cards once the event officially begins.