May 24 is a day of cultural importance
in many Orthodox Christian societies. It's the celebration of the Slavonic alphabet, credited to Sts. Cyril and Methodius.
From the article:
The Alphabet created by Cyril and Methodius became the basic one for all Slavonic languages and defined the spiritual and cultural development of many ethnicities, Vadim Leonov, archpriest and lecturer at the Sretenka Seminary said in an interview with the “Voice of Russia."