Three years after the fall of Constantinople (1453), Sultan Mehmed II — the Conqueror — turned his attention to central Europe. In June 1456 he laid siege to the fortress of Belgrade with an army of 60,000 men and a massive Hungarian artillery train. Pope Callixtus III called the universal Church to prayer and to the Rosary, ordering that the noon bells be rung in every parish of Christendom each day (a daily prayer summons that survives as the noon Angelus to this day).
The defenders, led by the Hungarian general John Hunyadi and by the 70-year-old Franciscan friar St. John Capistrano (who had personally rallied an irregular Crusader army of 30,000 peasants and minor nobility), prayed the Rosary continuously. On the night of July 21–22, after weeks of siege, Capistrano led the Crusaders in a sudden counter-attack across the river. The Ottoman army was routed by dawn; Mehmed II himself was wounded and barely escaped. Belgrade did not fall to the Ottomans for another 65 years. In thanksgiving, Pope Callixtus III extended the feast of the Transfiguration of the Lord (August 6) to the universal Church on the day the news reached Rome.