In 1263, a German priest, Peter of Praque arrived at Bolsena during his religious journey to Rome. He was very loving priest but he had difficultly accepting that the Host is the real Body of Christ.
At the tomb of St Christina, he was celebrating the Mass, and as he spoke the words of consecration, he saw Blood seep from the Host and over his hands and the altar.
At first, the priest tried to hide the Blood but he interrupted (ended) the Mass and went to the next town, Orvieto to see Pope Urban IV.
The Pope sent a person to investigate and found that the facts were correct. He ordered the Bishop of the diocese to bring the Host and the linen cloth with Blood stains to him and placed the Host in the reliquary in the cathedral. Also, the linen cloth is still there and exhibited in the Orvieto Cathedral.
Pope Urban IV informed the miracle to St Thomas Aquinas to write that facts, and declare this to be a true miracle. From these documents the Pope started the celebration of Corpus Christi in 1264.
Let you know that they removed the glass that covers the cloth once a year on the feast of Corpus Christi. They only do once a year to prevent the cloth from falling apart.