Saint Casimir's Day is observed on March 04, 2021. It is the anniversary of the death of Saint Casimir, the patron saint of Lithuania and Poland. St. Casimir is the patron saint of many Roman Catholic Churches in the United States which were established by Polish or Lithuanian immigrants.
St. Casimir's Day in many Roman Catholic parishes is marked with a special Mass said in his honor. His feast day, March 4, is commemorated optionally according to the Liturgical Calendar of the United States Conference of Bishops. It is, however, kept with great reverence in Roman Catholic parishes of ethnic Polish or Lithuanian origin
Surviving contemporary accounts described Prince Casimir as a young man of exceptional intellect and education, humility and politeness, striving for justice and fairness. Early sources do not attest to his piousness or devotion to god, but his inclination to religious life increased towards the end of his life. (With material from: Wikipedia)