Saint Casimir was born on October 3, 1458 in Wawel Castle in Krakow. He was the third child of thirteen and second son of the King of Poland and Grand Duke of Lithuania Casimir IV and Queen Elisabeth Habsburg of Hungary. Growing up, Saint Casimir and his brothers and sisters would often travel with the King and Queen in trips to Lithuania and by all accounts were part of a very loving family. Starting at the age of 9, Saint Casimir and his brother Vladislaus (who would later be Vladislaus II, King of Hungary) began to be tutored and education by a Polish priest named Jan Dlugosz. Father Jan taught them deeply, teaching Latin, German, law, history, and literature. Though this spiritual formation and education was critically important to young Casimir's life, the priest was very strict. In addition to teaching conservative topics like morality, ethics and religious devotion, Father Jan would also subject the two young boys to corporal punishment when they did not behave. This education paid off though, as Saint Casimir's skills in speaking were noted by Jakub Sienienski, the Bishop of Kujawy in 1470.
The life of everyone in the region soon was plunged into war and famine. In 1457 the uncle of Saint Casimir, Ladislaus the Posthumous, King of Hungary and Bohemia died suddenly. King Casimir IV hoped to advance his claims in Hungary and Bohemia but was slowed because of his participation in the Thirteen Year's War (a war caused by Prussian cities declaring independence from the Teutonic Knights and asking King Casimir IV for help). Seeking to take advantage of the situation, Hungarian nobles elected Matthias Corvinus as their King. King Casimir IV decided to install Saint Casimir on to the throne in Hungary. Poland gathered an army of 12,000 men to bring against Matthias Corvinus and his hired mercenary army. When they realized Matthias had gathered 16,000 men, the army retreated and began to suffer from food shortages, illness, and cold temperatures from the coming winter. Fearing for his safety, King Casimir IV sent his son back closer to the Polish border before the two army's agreed on a one year truce in 1472.
Saint Casimir felt great shame and sorrow for having to leave the army for his safety. Though the Polish kingdom portrayed the incident as Saint Casimir being a savior protecting the land from a godless tyrant and his invading pagan army, the incident had a profound effect on the still young Saint Casimir. Surviving sources of his life tell that Saint Casimir led a very pious life, always striving for justice, fairness and humility. He could often be found kneeling at the gate of the Church before dawn, waiting for a priest to come and open them so he could go inside and pray. After completing his education at 16, he began spending more time with his Father. The King tried to have him marry Kunigunde of Austria, the daughter of Emperor Frederick III but young Casimir turned down the proposal instead seeking to remain celibate.
In May of 1483, Saint Casimir completed an especially strict fast and began to suffer from tuberculosis. He joined his father in Vilnius where he took over some of the duties of the late Bishop and Vice-Chancellor of the Crown Andrzej Oporowski. As his health continued to worsen, a court physician suggested that he have sexual relations with a woman in an effort to cure his illness, a proposition Saint Casimir adamantly turned down. His Father rushed back to Lithuania in February 1484 when news reached him that his son's health had dramatically worsened. On March 4th, 1484, Saint Casimir died of Tuberculosis in Grodno. He was buried in Vilnius Cathedral.
Several miracles were then attributed to him, including a miraculous appearance to the Lithuanian army during the siege of Polotsk in 1518 where he showed the army where to safely cross the river and relieve the city. Pope Alexander VI in 1501, recognizing his miracles and the chapel he had been buried, granted an indulgence to anyone who prayed at the Chapel and any who contributed to the upkeep of it. Saint Casimir's brother, Sigismund I the Old began petitioning the Papacy to canonize his brother and in Pope Leo X appointed a commission to investigate. Pope Leo X canonized him, though the original documents no longer exist today, likely due to the sack of Rome in 1527. Pope Clement VIII issued Quae ad sanctorum that authorized Saint Casimir's feast on March 4 but only in Poland and Lithuania. Following a three day festival in Vilnius a cornerstone was laid for the new Church of Saint Casimir. It was written that when Saint Casimir's coffin was taken out of the crypt and elevated to the altar, a sweet smell filled the entire cathedral for three full days.
Bishop Woyna declared Saint Casimir the Patron Saint of Lithuania in 1607. In March of 1621, the Sacred Congregation of Rites granted the request to place his feast in the Roman Breviary and Roman Missal and a few years later, in 1636, Pope Urban VIII allowed the celebration with an octave. Many many years later, when Lithuanians found themselves as displaced war refuges in 1948, Pope Pius XII named Saint Casimir the Patron of Lithuanian youth.
A painting of Saint Casimir in Vilnius Cathedral is considered by many to be miraculous. According to the legends, the painter attempted to redraw one of the hands in a different place and painted over the old hand, but the old hand miraculously reappeared. Today the painting can be seen depicting Saint Casimir with two right hands.