Silver key pins are the current pins used to open holiday crates. Since they are buyable for a relatively low cost of 500 credits, I wouldn't call them rare. They could be used to enhance a trade though... Everybody loves savings!
If you have a key, place a crate in your guest room and walk up to it and click it. When you get a pop up asking if you would like to uncrate it, click uncrate
The crates that go with the silver keys are the different colored ornaments. They're called Holiday Ornament Crates. You can get them randomly from the NPCs (Esmeralda, Yeti, Maleficent, Captain Blackheart, Ned and Gator Waiter).
