“May”, kalendar page from Les Petites Heures d’Anne de Bretagne (The Little Hours of Queen Anne of Bretagne), by the Maître des Triomphes de Pétrarque. From Bibliothèque nationale de France, Paris (France).
It turns out that Boniface splits the office of Ice Saint with Saint Mamertus, depending on which three days are considered to be the “blackthorn winter”. Saint Mamertus is commemorated on May 11; Saint Pancras, on May 12; Saint Servatius on May 13; and finally Boniface on May 14. The so-called blackthorn winter, which presumably coincides with a brief cold spell, is a matter of folklore in a broad swath of Europe: Austria, Belgium, Croatia, Czechia, the Netherlands, France, Germany, Hungary, Poland, Slovakia, Slovenia, Switzerland, and Northern Italy. Interestingly, the blackthorn winter is considered to occur in May in these countries, but in England.
Depending on the country, the principal days are either May 11–13, or May 12–14; thus, from country to country, it may include either Mamertus or Boniface. And in some countries, the mini-season is capped by Cold Sophia—Saint Sophia, whose commemoration is May 15.
Evidence is inconsistent as to whether there really is a brief spell of colder weather at this time. On the other hand, at least in England, the white of the blackthorn blossom has the appearance of snow or hoar frost, so that may have led to humorous observations that winter had returned.
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