Saints Cyril and Methodius, Equals of the Apostles, and Enlighteners of the Slavs came from an illustrious and pious family living in the Greek city of Thessalonica. St Methodius was the oldest of seven brothers, St Constantine [Cyril was his monastic name] was the youngest. At first St Methodius was in the military and was governor in one of the Slavic principalities dependent on the Byzantine Empire, probably Bulgaria, which made it possible for him to learn the Slavic language. After living there for about ten years, St Methodius later received monastic tonsure at one of the monasteries on Mount Olympus (Asia Minor).
Source: Church Calender