The Ice Saints are St. Mamertus, St. Pancras, and St. Servatius. They are so named because their feast days fall on the days of May 11, May 12 and May 13 respectively, which often saw the last cold snap of the season. From Wikipedia
Low-pressure systems are driving a dramatic cooldown across the country, with severe weather warnings in place and unseasonal frost and snow complicating an ongoing drought.