The rule of thumb to only eat oysters in months with an "r" is at least 4,000 years old. This rule is based on the belief that oysters are more likely to be contaminated with bacteria during the warmer months of the year (May through August). This belief is thought to have originated in ancient Rome, where it was observed that oysters were more likely to cause food poisoning during the summer months. The rule of thumb was then passed down through the centuries and is still followed by many people today.

While there is some scientific evidence to support the rule of thumb, it is important to note that it is not foolproof. Oysters can still be contaminated with bacteria during the cooler months of the year, and they can also be safe to eat during the warmer months. The best way to ensure that oysters are safe to eat is to purchase them from a reputable source and to cook them thoroughly before eating them.