Cyril and Methodius invented the Slavonic alphabet in the ninth century, thus not only converting the Slavs, but also translating much ancient literature, some of which otherwise would have been lost. That literature includes some intriguing Old Testament Pseudepigrapha.
In various countries and churches their saint's day is celebrated annually on three different dates. For more on them, their day, and Old Church Slavonic literature, start here and follow the many links.
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