From now until August 20, you might find special playing cards and decks in
various locations around the game. The goal is to get a deck, and complete it
by finding all the cards that go in that deck, before the event ends.
There are three decks in total -- easy, medium and difficult -- to complete.
All cards and all decks are freely tradable -- any player may sell, give, or
trade their cards to any other player.
The easy and medium decks should not be too difficult to figure out, if you
actively try. The difficult one is probably going to involve some
crowdsourcing. Even if you figure out the theme, it's still not going to be
all that direct for a single player to find all the cards on their own. Get
together with your friends list, SS, and houses to exchange information.
When the event is over, you can trade in each type of deck once per player for
their rewards which, no I'm not telling you, but get better and better the
more complete the deck is. Or, keep the deck as a trophy. Your call.