Saint Casimir was the son of King Casimir IV of Poland. A zealous devotee to the Blessed Mother and the Eucharist, Casimir was especially heroic in the virtues of chastity and care for the poor. When his father was away in Lithuania, he ruled in his father's stead with great justice and prudence. He died in 1484 of tuberculosis.

Saint Casimir of Poland was nick-named "The Peacemaker" by his fellow countrymen. The third born of thirteen children, he was devout from a very young age. He was appalled by "softness" or self-indulgence to the point where he gave his life to penance and devotion. He would oftentimes put his bed on the ground and get up in the middle of the night to pray and meditate on the passion of the Christ.

The nobles of Hungary were not happy with their King, Matthias Cornvius. In 1471 they begged Saint Casimir's father, the King of Poland, to replace their King with his son. Casimir's father sent Casimir to the frontier as the head of an army. He obeyed his father, however, once he got there he returned home after he learned that his underpaid soldiers were deserting, Matthias was building a large army, and the Pope advised his father not to start a war. Casimir's Father was not happy and put his son in confinement. After that brief stint, Casimir returned to his studies and prayer.

His lungs failed at the age of 23 and he passed away in 1484. Saint Casimir was canonized in 1521 after miracles were reported at his tomb. In images, he is often seen holding a lily, which signifies his purity and innocence.