In an effort to limit the influence of encroaching Germanic neighbors, Czech Prince Rostislav in 863 invited Byzantine monks and brothers Cyril and Methodius of Thessalonica to come to the Czech lands to preach the gospel in the Slavonic language (a predecessor of Czech).
To spread the Gospel among the Slavs, they created the Cyrillic alphabet and translated the Bible and liturgical books into Slavonic, which at that time had no written form. The two brothers are considered the founders of Slavic literature and are sometimes known as the “apostles of the Slavs.”
The holiday is observed on July 5 because it coincides with the date of the Catholic feast day of Cyril and Methodius that was created by the Pope in 1880.
On this day every year thousands of Czech Roman Catholics gather at the Basilica of the Assumption of the Virgin Mary and Saints Cyril and Methodius in the south Moravian town of Velehrad, once the seat of the Great Moravian Empire, where they arrived in 863.