Saint Casimir, born in 1458 in Poland, was a prince known for his deep faith, charity, and commitment to living a life of purity and service to God. The son of King Casimir IV of Poland, he chose a life of celibacy, focusing on prayer, penance, and the welfare of the poor. He worked to strengthen Christianity in the region and was admired for his moral integrity. St. Casimir died at a young age, at 25, in 1484, likely due to illness. He is the patron saint of Poland, Lithuania, and youth. His feast day is celebrated on March 4th.

The Canadian Conference of Catholic Bishops (CCCB) has decreed that this is a Holy Day of Memorial (Optional) to be observed in Canada.