Casimir, the second son of Casimir III, King of Poland was born A. D. 1458. From the custody of a most virtuous mother, Elizabeth of Austria, he passed to the guardianship of a devoted master, the learned and pious John Dugloss. Thus animated from his earliest years by precept and example, his innocence and piety soon ripened into the practice of heroic virtue.
At the age of twenty-five, sick of a lingering illness, he foretold the hour of his death, and chose to die a virgin rather than take the life and health which the doctors held out to him in the married state. In an atmosphere of luxury and magnificence the young prince had fasted, worn a hair-shirt, slept upon the bare earth, prayed by night, and watched for the opening of the church doors at dawn. He had become so tenderly devoted to the Passion of Our Lord that at Mass he seemed quite rapt out of himself, and his charity to the poor and afflicted knew no bounds. His love for our blessed Lady he expressed in a long and beautiful hymn, familiar to us in our own tongue.
The miracles wrought by his body after death fill a volume. The blind saw, the lame walked, the sick were healed, a dead girl was raised to life. And once the Saint in glory led his countrymen to battle, and delivered them by a glorious victory from the schismatic Russian hosts.
One hundred and twenty-two years after his death the Saint’s tomb in the cathedral of Vienna was opened, that the holy body might be transferred to the rich marble chapel where it now lies. The place was damp, and the very vault crumbled away in the hands of the workmen; yet the Saint’s body, wrapped in robes of silk, was found whole and incorrupt, and emitted a sweet fragrance, which filled the church and refreshed all who were present. Under his head was found his hymn to Our Lady, which he had had buried with him. The following night three young men saw a brilliant light issuing from the open tomb and streaming through the windows of the chapel.
Lives of the Saints, by Alban Butler, Benziger Bros. ed. 
St Casimir (1458 – 1484)