Allegedly, it was on 14th February 270 AD that a Christian priest in Rome was executed on the orders of the then emperor. The villain of the piece was Claudius II also known at Claudius the Cruel. Claudius was grappling with shortage of manpower in the Roman Army and came to believe that it was a result of men not wanting to be parted from their wives while on service. He therefore banned all marriages and engagements in Rome. Valentine defied this ban and continued to perform marriages. An alternative theory is that because Valentine was a Christian and Rome was still largely Pagan he was accused of subversive activities.
Not surprisingly, when this became known to the authorities Valentine was in big trouble. He was dragged before the prefect of Rome who sentenced him to death by clubbing followed by a beheading. The legend suggests that he left a farewell note to the daughter of his gaoler who he had befriended signing it "From Your Valentine". He was buried along the Via Flaminia and a later Pope, Julius I, erected a basilica over his grave. Archaeologists have found remains.
The problem with legends is that they are notoriously unreliable. Early Catholic records suggest that there are three saint Valentines all of whom were martyrs. Apart from the Valentine mentioned above there was one from Terni in Italy and one from a Roman province in Africa.