Cyril and Methodius invented the Slavonic alphabet in the ninth century, thus not only converting the Slavs, but also preserving much ancient literature that otherwise would have been lost. That literature includes some intriguing Old Testament Pseudepigrapha.
Today the Roman Catholic Church, the Anglican Communion, and (sometimes) the Lutheran Church commemorate them. Other traditions do so on 24 May or 5 July. Follow the links for details.
Oh yes, and happy belated Valentine's Day. Some relevant PaleoJudaica posts are here, here, here, here, and here.
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