Today according to the Bulgarian Orthodox Church calendar we commemorate the memory of probably the greatest saint of Bulgaria, second in saintship after St. John of Rila
St. King Boris was the ruler of the First bulgarian Empire (852-889). He received holy baptismal in year 864, receiving the Christian name Michael (Mihail) receiving his Christian name from his godfather, Emperor Michael III.
During his holy reign he has established mass Christianization of Bulgaria, where the traditional ancient pagan traditions and belief in fake gods like Tangra were abolished completely.
St. Tsar Boris has secured the bulgarian church an autocephalousy, he also received the saints Cyril and Methodius, when they were banished from Great Moravia.
Our saint king has secured a refuge for st. Cyril and Methodius and provided them with assistance to develop the Slavonic alphabet and literature.
After he abdicated in 889, his eldest son and successor tried to restore the old pagan religion but was deposed by Boris I. During the Council of Preslav which followed that event, the Byzantine clergy was replaced with Bulgarian and the Greek language was replaced with Old Bulgarian as an official language of the Church and the state.
In 889 Boris abdicated the throne and became a monk. His son and successor Vladimir attempted a pagan reaction, which brought Boris out of retirement in 893.
Vladimir was defeated and blinded, and Boris gathered the Council of Preslav placing his third son, Simeon I of Bulgaria on the throne, threatening him with the same fate if he too apostatized.
Boris returned to his monastery, emerging once again in c. 895 to help Simeon fight the Magyars, who had invaded Bulgaria in alliance with the Byzantines. After the passing of this crisis, Boris resumed monastic life and led a holy life until he pass out in year 907.
Here is the daily troparion assigned by the Bulgarian Orthodox Church sang at the churches today (the text is translation from Bulgarian):
St. Boris-Michael, prince of Bulgaria, Troparion
Full of the fear of God, and enlightened by holy baptism, thou becamest a habitation of the Holy Spirit,
O right-believing King Boris; and having established the Orthodox Faith in the land of Bulgaria,
and set aside the scepter of kingship,
thou madest thine abode in the wil derness,
didst flourish in ascetic struggles, and found grace before the Lord.
And now, standing before the throne of the Most High, pray thou, that He grant unto us who entreat thee salva tion for our souls.