Oysters are best consumed in months containing the letter R (DecembeR, JanuaRy, FebRuaRy). This is because they spawn during the summer months and become soft and fatty as they grow. The cold water makes the oysters plump and luscious for harvesting in the winter. Reject those that do not have tightly closed shells or that don't snap shut when tapped.
The smaller the oyster is, the younger and more tender it will be. Fresh, shucked oysters are available at grocery stores and should be chubby, uniform in size, have good color, smell fresh and be packaged in clear - not cloudy - liquid.