According to tradition, during the summer of 1263 a Bohemian priest by the name of Peter went on a pilgrimage to Rome in order to confront the doubts he was having about his faith and the resulting crisis in his vocation. His faith was renewed after praying at the tomb of the apostle Peter. During his journey home, he happened to celebrate Mass in the Church of Santa Cristina in Bolsena. During the liturgy of the Eucharist, the host miraculously began to bleed. The priest was profoundly perplexed and immediately wrapped the host in the corporal and sacred linens before leaving the sanctuary. Coming to terms with what he had witnessed, he went to Orvieto where Pope Urban IV was residing and he told him about the incident. In order to investigate the matter further, the Pope immediately sent Bishop Giacomo to the place of the event. When he returned, the relics of the miracle were carried in procession by the entire papal court throughout the streets of the city, where thousands of the faithful had gathered. Tied to these miraculous events is also the construction of the Duomo of Orvieto where the corporal stained with blood is kept. On 11 August 1264 the miracle was officially recognized by Pope Urban IV with the bull “Transiturus de hoc mundo”, promulgated in Orvieto. A detail of a work by Raffaello Sanzio, known as the “Mass of Bolsena” (Stanze of Raphael, Vatican Museums), is reproduced on the postal minisheet which commemorates the 750 years since this miraculous event.