As is the way with myths, they are handed from generation to generation, reinforced with each telling, until they become an almost inherent part of the culture in which they grew. So far as the United States is concerned, this is not true. Under commercial raising and harvesting conditions, oysters and other shellfish are safe and good to eat any month of the year. When this occurs, people are warned against gathering and eating these particular shellfish. The cardinal rule is that any commercially available shellfish is non-toxic and safe to eat.
Oysters are an aphrodisiac.
The idea of eating oysters for their aphrodisiac qualities, which has been around for a long time, is basically untrue. Shellfish that die before being cooked should not then be cooked and eaten. People have been warned not to eat clams, mussels, crabs, lobsters and other shellfish unless they are alive when cooked. From the standpoint of flavor, this is a good suggestion, but shellfish don't become toxic when they die. When shellfish die, their digestive glands break down, releasing digestive enzymes that begin digesting the flesh of the animal.