There used to be THIS IS A LINK some truth to this belief. Before the advent of refrigeration, oysters - any seafood for that matter - were more likely to spoil during the hot summer months, which by pure chance do not have an "r" in their names. And a run-in with a spoiled oyster is to be avoided for sure!! But today, oysters are kept icy cold from ocean to plate, so these worries are a thing of the past.