Oysters are edible all year long, but they tend to be less plump and tasty during their spawning season, from May to August (months with no R in their names). To assure freshness, use only oysters that are tightly closed.
Scrub the oysters with a stiff brush to remove any sand. To open an oyster, grasp it firmly in a thick towel with the flatter shell facing up. Push the tip of an oyster knife or a knife with a short strong blade—about 1/2 inch into the hinge, or pointed side, of the oyster. Twist the knife to pry the shells apart, then slide the knife around the inside of the upper shell to cut the muscle that holds the shell to the oyster. Pull off and discard the upper shell. Pass the knife under the oyster to cut it from the bottom shell, and flick out any bits of broken shell with the tip of the knife.
Serve the raw oysters on a half shell with a little lemon juice and black pepper or dip them into a mixture of ketchup, horseradish sauce, lemon juice, and liquid red pepper sauce.