Wikipedia:
The first representation of Saint Valentine appeared in the Nurembrg Chronicle(1943); alongside the woodcut portrait of Valentine, the text states that he was a Roman priest martyred during the reign of Claudius II, known as Claudius Gothicus. He was arrested and imprisoned upon being caught marrying Christian couples and otherwise aiding Christians who were at the time being persecuted by Claudius in Rome. Helping Christians at this time was considered a crime. Claudius took a liking to the prisoner -- until Valentine tried to convert the Emperor -- whereupon this priest was condemned to death.
Happy Valentines Day!
