Mamertus, the first of the Ice Saints
Mamertus, the first of the Ice Saints is held on May 11. This event in the second decade of the month May is annual. 
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Mamertus is the first of five saints whose feast days have, over five centuries or so, coincided with a mid-spring cold spell often enough to warrant a foreboding nickname: the ice saints. His feast day and those of Sts.
The Ice Saints are St. Mamertus (or, in some countries, St. Boniface of Tarsus), 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, known as the blackthorn winter in Austrian, Belgian, Croatian, Czech, Dutch, French, German, Hungarian, North-Italian, Polish, Slovak, Slovene and Swiss folklore.
Similar holidays and events, festivals and interesting facts
Victory Day in Poland and Czech on May 8 (Narodowy Dzień Zwycięstwa in Poland established in 2015; Liberation Day in Czech Republic);
Day of Patriots and Military in Hungary on May 21
Armed Forces Day in Croatia on May 28
Children's Day in Hungary on May 28 (celebrated on the last Sunday in May);
Parliament Day in Croatia on May 30 (Statehood Day);
Sea kayaking in Croatia on June 1 (June to September);
Teacher's Day in Hungary on June 4 (celebrated on the first Sunday in June);
National Unity Day in Hungary on June 4 (dedicated to the anniversary of the Trianon Treaty)