24 May is known as Saints Cyril and Methodius Day or the Day of Slavonic Literature and Culture.
Two Byzantine missionaries, Cyril and Methodius, used the Greek alphabet to created the Cyrillic script. The older Glagolitic alphabet was incorporated into it as well. The translations of the biblical texts became first samples of the Cyrillic writings. Cyril and Methodius are venerated in the Orthodox Church as saints.
Cyrillic is currently used exclusively or as one of several alphabets for more than 50 languages, notably Belarusian, Bulgarian, Kazakh, Kyrgyz, Macedonian, Montenegrin (spoken in Montenegro; also called Serbian), Russian, Serbian, Tajik (a dialect of Persian), Turkmen, Ukrainian, and Uzbek.
Source: www.britannica.com