Kaziuko mugė or Saint Casimir's Fair is a large annual folk arts and crafts fair in Lithuania, dating to the beginning of the 17th century. The fair is traditionally held in city's markets and streets on the Sunday nearest to 4 March, the anniversary of Saint Casimir's death. In Lithuanian, Kaziukas is a diminutive of Casimir. Today, Saint Casimir's fair also features music, dance, theater performances; it attracts tens of thousands of visitors and many craftsmen from across Lithuania as well as from neighbouring countries such as Latvia, Russia, and Poland. In recent years, the fair has expanded into other cities in Lithuania, Belarus, Poland.

Casimir Jagiellon was a prince of the Kingdom of Poland and of the Grand Duchy of Lithuania. The second son of King Casimir IV Jagiellon, he was tutored by Johannes Longinus, a Polish chronicler and diplomat. After his elder brother Vladislaus was elected as King of Bohemia in 1471, Casimir became the heir apparent. At the age of 13, Casimir participated in the failed military campaign to install him as King of Hungary. He became known for his piety, devotion to God, and generosity towards the sick and poor. He became ill and died at the age of 25. He was buried in Vilnius Cathedral. His canonization was initiated by his brother King Sigismund I the Old in 1514 and the tradition holds that he was canonized in 1521.