This feast was originally the second commemoration of St. Leo the Great, who was the object of very marked devotion in the Middle Ages. It was celebrated on the anniversary of the removal of his body from the porch to the interior of St. Peter’s. Later it became the feast of Pope Leo II who approved the acts of the Sixth Ecumenical Council which condemned the heresy of those who asserted that Christ had only one will. In his brief pontificate he gave an example of earnest preaching and devotion to the poor. He died in 683.
(Source: The New Roman Missal, Rev. F. X. Lasance)