From Catholic.org:

Casimir (1461 – 1484) grew up in a world where his life was not his own. As a prince of Poland, the second son of King Casimir IV and Elizabeth of Austria, his life was scheduled to cement his father’s authority and increase Poland’s power. Casimir realized from an early age that his life belonged to someone else, but to a much higher King than his father. Despite pressure, humiliation, and rejection, he stood by that loyalty through his whole life.

…Though his father must have wondered about him, he must have seen and admired Casimir’s strength. He showed that he misunderstood this strength when he sent Casimir as head of an army to take over the throne of Hungary at the request of some nobles there. Casimir felt the whole expedition was wrong but was convinced to go out of obedience to his father. He could not help but feel at every step that it was disobedient to his other Father. So when soldiers started deserting, he was only too glad to listen to the advice of his officers and turn back home. His feelings were confirmed when he discovered that Pope Sixtus IV had opposed the move.

His father, however, was furious at being deterred from his plans and banished Casimir to a castle in Dobzki, hoping that imprisonment would change Casimir’s mind. Casimir’s commitment to what he believed was right only grew stronger in his exile and he refused to cooperate with his father’s plans any more despite the pressure to give in. He even rejected a marriage alliance his father tried to form. He participated in his true King’s plans wholeheartedly by praying, studying, and helping the poor.

He died at the age of 23 in 1484 from lung disease. He was buried with his favorite song, a Latin hymn to Mary called “Omni die dic Mariae” which we know as “Daily, Daily Sing to Mary.” Because of his love for the song, it is known as the Hymn of St. Casimir though he didn’t write it.