Oyster eating is a year-round affair thanks to modern day food preservation. It's been codified since 1599 that you should only eat oysters in months that have an "r" in their name. This leaves out the months of May through August -- some of the hottest months of the year in oyster-rich regions. Although this recommendation is not as relevant in today's modern food environment, it follows common sense.
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Why Summer is in Question
Eating oysters or any other shellfish and mollusks that have hit a warm, but not cooked temperature can be toxic to your system. A warm oyster may be more likely to carry a foodborne pathogen.
Summer months are also the time that oysters spawn. A spawning oyster doesn't make for good eating because its plump, briny body becomes thin and milky. Oysters are weaker when they spawn and are more susceptible to disease.
The water in which the oysters reside are also less optimal during the summer months. Oysters thrive on plankton, that can become riddled with bacteria in warm summer water.
No Longer a Valid Recommendation
You only need heed the "r" month warning if you're hunting oysters yourself. Commercial oyster farmers take precautions that keep their oysters safe year-round. Farmed oysters don't sit on boats for hours and many farms use non-spawning oysters -- called triploids. Even coveted types -- such as Blue Point oysters, which hail from locations along the Long Island Sound -- are farmed and safe to consume year-round from trusted purveyors. Oyster hunting is strictly regulated by area, too, so heed local recommendations and warnings if you're picking your own.
Selecting an Oyster
Regardless of when you choose oysters for home cooking, ensure you have quality specimens. Oysters' shells should be tightly closed and should have no rancid smell. Ask your fish monger to show certification that the oysters are from certified fishing waters.
Do heed food safety when consuming oysters. Raw oysters should be avoided by those with compromised immune systems, including the elderly, small children and pregnant women.