Casimir Jagiellon (Latin: Casimirus; Lithuanian: Kazimieras; Polish: Kazimierz; 3 October 1458 – 4 March 1484) was a prince of the Kingdom of Poland and of the Grand Duchy of Lithuania.

The second son of King Casimir IV Jagiellon, he was tutored by Johannes Longinus, a Polish chronicler, diplomat, and priest.

In Vilnius, his feast day is marked annually with Kaziuko mugė (a trade fair) held on the Sunday nearest to 4 March, the anniversary of his death.

[5] Długosz noted Casimir's skills in oratory when he delivered speeches to greet his father returning to Poland in 1469 and Jakub Sienienski, the Bishop of Kujawy, in 1470.

[5] Prince Casimir's uncle Ladislaus the Posthumous, King of Hungary and Bohemia, died in 1457 at the age of 17, without leaving an heir.

Matthias Corvinus managed to win over the majority of the Hungarian nobles, including the main conspirator Archbishop János Vitéz, and the Polish army did not receive the expected reinforcements.

[8] In December 1471, Prince Casimir, out of fear for his safety, was sent to Jihlava closer to the Polish border and that further eroded their soldiers' morale.

[9] Polish propaganda, however, portrayed him as a savior, sent by divine providence, to protect the people from a godless tyrant (i.e. Matthias Corvinus) and marauding pagans (i.e. Muslim Ottoman Turks).

Prince Casimir was also exposed to the cult of his uncle King Władysław III of Poland who died in the 1444 Battle of Varna against the Ottomans.

This led some researchers, including Jacob Caro, to conclude that the Hungarian campaign pushed Prince Casimir into religious life.

[9] As his elder brother, Vladislaus II, ruled Bohemia, Prince Casimir became the heir apparent to the throne of Poland and Lithuania.

[9] In 1476, Prince Casimir accompanied his father to Royal Prussia where he tried to resolve the conflict with the Prince-Bishopric of Warmia (see War of the Priests).

King Casimir IV feared separatist moods and refused, but after settling the conflict in Prussia, moved to Vilnius.

In February 1484, the Polish sejm in Lublin was aborted as King Casimir IV rushed back to Lithuania to be with his ill son.

[14] Surviving contemporary accounts described Prince Casimir as a young man of exceptional intellect and education, humility and politeness, who strove for justice and fairness.

Marcin Kromer (1512–1589) said Casimir refused his physician's advice to have sexual relations with women in hopes to cure his illness.

[16] Other accounts say Casimir contracted his lung disease after a particularly hard fast or that he could be found pre-dawn, kneeling by the church gates, waiting for a priest to open them.

Casimir showed where Lithuanian troops could safely cross the Daugava River and relieve the city, besieged by the army of the Grand Duchy of Moscow.

The description lacks specifics, such as date or location, but most likely refers to the Lithuanian victory in 1519 against Russian troops that raided the environs of Vilnius, and not the more popular story of the Siege of Polotsk.

[21] In 1513, Andrzej Krzycki wrote a poem mentioning numerous wax votive offerings on Casimir's grave.

[23] After repeated requests, in November 1517, Pope Leo X appointed a three-bishop commission and later sent his legate Zacharias Ferreri to investigate.

Research of Zenonas Ivinskis and Paulius Rabikauskas showed that there is no documentary proof that he issued a papal bull canonizing Casimir[26] but many important documents were lost during the Sack of Rome (1527).

[30] The Sacred Congregation of Rites refused the request[30] but on 7 November 1602 Pope Clement VIII issued a papal brief Quae ad sanctorum which authorized his feast sub duplici ritu on 4 March but only in Poland and Lithuania.

[30] In March 1636, Pope Urban VIII allowed the celebration of the feast of Casimir with an octave (duplex cum octava) in the Diocese of Vilnius and the Grand Duchy of Lithuania.

On 28 September 1652, Pope Innocent X allowed a fest of transfer of relics of Casimir on a Sunday following the Assumption of Mary.

[40] In 1655, before the Battle of Vilnius during the Deluge, the relics were removed most likely by Jerzy Białłozor and hid by Cyprian Paweł Brzostowski and later by the Sapiehas in the Ruzhany Palace.

[44] There are several recorded instances when Casimir's relics were gifted to prominent figures and societies: to musicians' confraternity at San Giorgio Maggiore, Naples in early 1650s,[48][49][50] to King John III Sobieski and Cosimo III de' Medici, Grand Duke of Tuscany[n 1] in October 1677,[50] to the Sodality of Our Lady of the Jesuit academy in Mechelen[47] and the Order of Malta in October 1690,[51] to Queen Maria Josepha of Austria in February 1736,[35] to Cistercian abbot Sztárek Lajos of Cikádor Abbey [hu] in 1860.

In 1945, the College of Saint Casimir was established in Rome to educate Lithuanian priests who fled west after World War II.

Sculptures of Casimir, among other canonized royals, can be found in San Ferdinando, Livorno, Italy and Metropolitan Cathedral, Mexico City.

Stained glass windows with Casimir can be found at the Cathedral Basilica of St. Joseph in San Jose, California, and at the Church of St. Peter in Chevaigné, France.