Saint Casimir Jagiellon (1458 – 1484) was a prince of the Kingdom of Poland and of the Grand Duchy of Lithuania. Second oldest son of King Casimir IV, he was tutored by Johannes Longinus, a Polish chronicler and diplomat.

After his elder brother Vladislaus was elected as King of Bohemia in 1471, Casimir became the heir apparent. At the age of 13, Casimir participated in the failed military campaign to install him as King of Hungary.

He became known for his piety, devotion to God, and generosity towards the sick and poor. He became ill and died at the age of 25. He was buried in Vilnius Cathedral and his cult grew.

His canonization was initiated by his brother King Sigismund I the Old in 1514 and the tradition holds that he was canonized in 1521.

The age of Protestant Reformation was not conducive to the cult of saints. St. Casimir's cult saw a resurgence in the 17th century when his feast day was confirmed by the pope in 1602 and the dedicated Chapel of Saint Casimir was completed in 1636.

St. Casimir became a patron saint of Lithuania and Lithuanian youth. In Vilnius, his feast day is marked annually with Kaziuko mugė (a trade fair) held on the Sunday nearest to March 4, the anniversary of his death.

There are more than 50 churches named after St. Casimir in Lithuania and Poland, including Church of St. Casimir, Vilnius and St. Kazimierz Church, Warsaw, and more than 50 churches in Lithuanian and Polish diaspora communities in America.

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