Saints Cyril and Methodius Day is observed in the Czech Republic and Slovakia every July 5 as a public holiday, honoring the two brothers who are co-patron saints of these countries. The day commemorates their contributions as principal Christian missionaries among the Slavic peoples in Pannonia and Great Moravia, where they introduced Orthodox Christianity and created the Glagolitic alphabet, the oldest known Slavic alphabet. The missionaries were sent to Great Moravia in 862 at the request of Prince Rastislav, who sought support from the Byzantine Empire over Roman Catholic missionaries. While the Roman Catholic Church celebrates their feast day on February 15, the observance on July 5 is significant in Slovakia and the Czech Republic, reflecting the historical and cultural impact of Cyril and Methodius in the region.