Valve has dropped what is being interpreted as a hint that the Steam Summer Sale will kick off this Wednesday.
PCGamesN reports that the Steam Trading Card beta will be ending on June 26th at 7pm UK time. At that point Steam Trading Cards will be open to everyone, with Valve saying of the move:
"Making trading cards available to all Steam users is a step towards an event we think everyone can be excited about."
Steam's Summer Sale normally kicks off around the end of June or beginning of July and is, according to PCGamesN, often associated with some sort of meta-game – meaning a Trading Card tie-in would make perfect sense.