Today is the feast day of St. Cyril, a 4th Century man who served Bishop of Jerusalem for more nearly forty years. My church does not honor or celebrate the Saint's days specifically, but I have felt a leading to learn about those giants of the faith that came before (waaaay before) me. And St. Cyril is one of those giants.
Cyril was named one of the three Doctors of the Eastern Church for his great contributions to what we know of today as orthodox theology. He was a major force in the Nicene faction, in the fight against Arianism, the heresy that denied the divinity of Christ. The description of the Lord as being "of one substance with the Father" in the Nicene Creed is a result of the theological battles over this issue. Cyril was an important teacher of the orthodox faith that came from the Council of Nicaea.
In addition to this, St. Cyril introduced the liturgical innovation of celebrating Holy Week and Easter.