Saint Mamertus, Saint Pancras, and Saint Servatus
Never pass without a frost.
Now there is great wailing and gnashing of teeth among those who - beguiled by the warm temperatures heretofore - have planted their tomatoes, only to see the temperatures drop and their proud plants droop and die.
About this time in May, the benign weather we have been enjoying turns cold [at least it does here in the Smallest State]. The saints whose feast days fall during that time are called the "Ice Saints" or the "Chilly Saints", and prudent gardeners believe that nothing is safe from frost until these days have passed.
The tradition seems to have been confined to northern and middle Europe, especially the areas in and around France, Germany, and later, Sweden. Most often, the number is three, as named in the couplet above, but some places also add Saint Boniface and Saint Sophia (as "Cold Sophie") to the original trio.
So, if you've already set out your tender plants, protect them. If you've put away your sweaters, get them out again. Stow the barbecue for a few days and return to hearty, winter-type fare. As this is mostly a Germanic tradition, this would be a good time to eat wurst und schnitzel, maybe a little kraut, and drink bier und schnapps (and make voopie mit beautiful frauleins, according to Baron Sam von Schpamm). Five days of schnapps, and Kalt Sophie won't seem so kalt!
Today is the feast of Saint Mamertus (died c. 477), Archbishop of Vienne in France. He introduced the Minor Rogations, days of fasting and prayer imploring God's mercy and protection, and a good harvest. Of his dispute with Pope Hilarius, please read the entry in the Catholic Encyclopedia.