“Only eat oysters in months with an 'R.'" This old wives tale carries more truth than you might expect. Months without the letter "R" in their names (May through August) coincide with summer in the Northern Hemisphere and the advice dates back at least to 1599, when it appeared in an English cook book (Henry Buttes' "Dyets Dry Dinner,") though some historians say it goes back as far as the days of the Roman Empire.
In pre-refrigeration days in warm months, oysters and shellfish would spoil easily and bad toxins could rapidly multiply in warm summer seas and rivers.
Summer is also the spawning season for oysters and so their energy is used to fuel reproduction and the meat can become thin.
These days this old adage no longer really applies to commercially farmed oysters but if you fish for them yourself – it absolutely still applies!