From Wikipedia, Saints Cyril and Methodius:
"Saints Cyril and Methodius Day" is a holiday, usually celebrated on 11 May (24 May) in countries which observe Eastern Orthodox tradition, and on 5 July in countries that observe Roman Catholic tradition. It commemorates the creation of the Slavic Glagolitic and Cyrillic alphabets by the brothers Saints Cyril and Methodius. There is evidence that the Cyrillic alphabet was not created by Cyril and Methodius but by their pupil, Clement of Ohrid, but the alphabet bears Cyril's name nonetheless, and the evidence is still not enough to disprove that.) The celebration also commemorates the introduction of literacy and the preaching of the gospels in the Slavonic language by the brothers.A note of the earlier celebration, with links to background on Slavonic pseudepigrapha etc., is here.