Solution: Like poor months for oysters, it's said - NYT Crossword

Back before refrigeration and rapid transit, you were not supposed to eat oysters and other shellfish in the heat, lest they kill you or make you wish they had. The summertime was also when oysters made more oysters, so it was considered respectful to give them some privacy. The rule of thumb was to watch out for the months that were RLESS, lacking the letter “r” in their name, something peculiar to May through August. These days, of course, science has removed most of the risks of spoilage with dry ice and jet travel, as well as most of the fun from the lives of oysters with asexual triploid reproduction and year-round farming, so they probably want us to eat them, really.