In 862, at the request of Prince Rastislav of Great Moravia (present day Slovakia and the Czech Republic including parts of Hungary, Poland, Austria, Slovenia, Croatia, Serbia, Ukraine and Germany), the Patriarch Photius sent the brothers Cyril and Methodius to bring Christianity to Moravia.
These brothers are credited with inventing the Glagolitic Alphabet, the first used to translate the Old Church Slavonic language. This was then simplified into the Cyrillic Alphabet (named after Cyril) which is still in use today.
If you are of Slavic origin, May 11 is your day to celebrate. This, according to the Greek Orthodox Calendar. The Romans and Anglicans celebrate this on February 14, the date of Cyril’s death.