I did a sloppy job of writing this. It’s not that these 3 days are cold, necessarily, but that after they’ve passed there’s no danger of frost.
Happy Mamertus’ Day, everybody!
“Mamertus, Pancras, and Gervais were three early Christian saints. Because their feast days, on May 11, 12, and 13, respectively, are traditionally cold, they have come to be known as the Three Chilly Saints.”
On May 14 it’s okay to plant your tomatoes.