Historically it has been safer to eat oysters in the colder months
There is an old adage that you should only eat oysters in months that have the letter “R” in their names. This adage is specifically for the names of months in English and French, and also specifically for the northern hemisphere where these would be the cooler months, September through April.
The reason is simple: before refrigeration oysters would spoil in transit during the warmer summer months (months without an “R” in their names). So, unless you lived near the water with the oysters, there was a greater chance that by the time the oysters got shipped to you they would have spoiled.