What? You were expecting St. Valentine? Come on, everybody knows Valentine was dropped years ago from the general Roman calendar. ;-)
These apostles to the Slavs are credited with, among other things, inventing the Cyrillic alphabet, so they could teach the Slavs to read the Gospels.
Saints Cyril and Methodius (Greek: Κύριλλος και Μεθόδιος, Old Church Slavonic: Кѷриллъ и Меѳодїи[more]) were two Greek brothers born in Thessaloniki in the 9th century, who became missionaries of Christianity among the Slavic peoples of Great Moravia and Pannonia. Through their work they influenced the cultural development of all Slavs, for which they received the title "Apostles to the Slavs". They are credited with devising the Glagolitic alphabet, the first alphabet used to transcribe the Old Church Slavonic language, as well as the the Cyrillic alphabet. After their death, their pupils continued their missionary work among other Slavs. Both brothers are venerated in the Eastern Orthodox Church as saints with the title of "Equals to the Apostles". In 1880, Pope Leo XIII introduced their feast into the calendar of the Roman Catholic Church. In 1980, Pope John Paul II declared them co-patrons of Europe, together with Saint Benedict of Nursia.