The modern St. Valentine's Day celebrations are said to have been derived from both ancient Christian and Roman tradition. As per one legend, the holiday has originated from the ancient Roman festival of Lupercalis, a fertility celebration that used to observed annually on February 15. But the rise of Christianity in Europe saw many pagan holidays being renamed for and dedicated to the early Christian martyrs. Lupercalia was no exception. In 496 AD, Pope Gelasius turned Lupercalia into a Christian feast day and set its observance a day earlier, on February 14. He proclaimed February 14 to be the feast day in honor of Saint Valentine, a Roman martyr who lived in the 3rd century. It is this St. Valentine whom the modern Valentine's Day honors.
According to the Catholic Encyclopedia, there were at least three early Christian saints by the name of Valentine. While one was a priest in Rome, another was a bishop in Terni. Nothing is known about the third St. Valentine except that he met his end in Africa. Surprisingly, all three of them were said to have been martyred on 14th February.