My Dad loved clams. He would enjoy them raw or steamed or even fried. But, as much as he loved them, he said, "Don't eat shellfish during months without an "R" in their name."
So, apparently, May, June, July, and August are not good months to eat clams, oysters, shrimp or mussels.
I wish I had been aware enough to ask Dad "Why?" But, I was young and dumb and didn't really think about what he was saying.