St. Stanislaus, a Polish martyr, was the Bishop of Cracow. King Boleslaw II was outraging the whole kingdom by his acts of lust and cruelty. St. Stanislaus, finding all remonstrances useless, excommunicated him. By order of the king he was cut into pieces at the foot of the altar while celebrating high Mass on the Feast of the Apparition of St. Michael in the oratory of the Holy Archangel on the outskirts of Cracow. As, however, that day has been dedicated from the early Middle Ages to St. Michael, when the feast of the martyred bishop was introduced into the Calendar of the Universal Church by Clement VIII, it was kept in anticipation on the eve of his death.
(Source: The New Roman Missal by Father Lasance, p. 1693)