To most people today is the feast of St Valentine, but according to the Roman Calendar it is the feast of SS Cyril and Methodius, the Apostles of the Slavs. Their widespread cult is relatively recent in western Europe, dating really from the time of Pope Leo XIII. It became more extensive, and ranked as a Feast rather than a Memoria in the Novus Ordo when Pope John Paul II proclaimed them co-patrons of Europe with St Benedict in 1980.
There is an introduction to their lives, achievements and legacy here.
From that their importance in the history of Bulgaria is made clear, and, as aconsequence, in 1909 King Ferdinand I established the Order of SS Cyril and Methodius as the highest chivalric order of his kingdom. I hope to post more about the order and its insignia in the near future