St. Cyril and St. Methodius, the two missionary brothers who gave birth to the written Slovak language, are celebrated by the Eastern Catholic Church in May. (The exploits of the Brothers are detailed here.) But in the early 20th century the Roman Catholic feast day for the saints, held on July 5, took on a new importance.
After the formation of Czechoslovakia in 1918, Slovaks around the world struggled to defend their national identity. When the Czechs declared July 6 a public holiday, in honor of 15th century Protestant forerunner Jan Hus, Slovaks pushed for the feast day of Sts. Cyril and Methodius, July 5, as a national celebration as well as religious one. Today it’s celebrated in both Slovakia and the Czech Republic.