Happy Feast of the Transfiguration
The old Catholic Encyclopedia -- courtesy of www.newadvent.org -- says this:
"In 1456 Callixtus III extended the feast to the Universal Church in memory of the victory gained by Hunyady at Belgrade over the Turks, 6 August, 1456. Callixtus himself composed the Office."
Jesus, mercy! Mary, help! Amen.