Saint Casimir's Day is celebrated on March 4, the anniversary of the death of Saint Casimir, the patron saint of Lithuania and Poland. The Kaziuko mugė, a folk arts and craft fair, is traditionally held on the Sunday nearest to March 4.
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.