A good oyster is closed, heavy for its size and smells of the sea. Keep it cupped side down in the fridge under a damp cloth, never in water.
To open, wrap your hand in a folded tea towel, push the knife into the hinge and twist. Slide the blade under the top shell to cut the muscle.
Rock oysters are fine all year; the old rule about months with an R is about native oysters, which spawn in summer.