Saint Casimir (1458-1484) was the son of Casimir (Kazimieras), Grand Duke and King of Lithuania. He was born and grew up in Poland, lived in Lithuania staring from 1475 and was King's viceroy in Vilnius. Being a pious and sick person (was ill with tuberculosis) he was not a good politician. He was buried in the crypt of Vilnius Archcathedral. St Casimir's remains are guarded in the three coffins made of silver, oak and ebony. Only Vilnius Bishop Jurgis Matulaitis was a person who had once opened all three coffins and described the remains and reliquaries that were inside.

Pope Leone X canonised Saint Casimir in 1521, unfortunately, his unexpected death became an obstacle for the preparation of necessary documents. Pope Clemens VIII confirmed his cult as saint for the second time in 1602. The third time Pope Urban VIII proclaimed St Casimir the Heavenly Patron of Lithuania in 1636. The fourth time Pope Pius XII declared St Casimir the Heavenly Patron of Lithuanian youth in 1948. St Casimir's remains were taken from the Archcathedral twice: the first time after Russian occupation in 1655 and the second time in 1952 10 09 when the remains were transferred to the church of SS Peter and Paul. This decision was taken after the Soviet authorities had decided to close the Archcathedral and equip it for tractor manufacture. Stalin granted the permission to the representatives of Lithuanian Catholic church for transfer of the remains of St Casimit to SS Peter and Pauls' church. The remains were brought back to the Archcathedral in 1989 03 04. In the night of March 4 th four priests placed the coffin with the remains of St Casimir above the altar in the royal chapel of St Casimir.