From the CATHOLIC ENCYCLOPEDIA:
At least three different Saint Valentines, all of them martyrs, are mentioned in the early martyrologies under date of 14 February. One is described as a priest at Rome, another as bishop of Interamna (modern Terni), and these two seem both to have suffered in the second half of the third century and to have been buried on the Flaminian Way, but at different distances from the city. In William of Malmesbury's time what was known to the ancients as the Flaminian Gate of Rome and is now the Porta del Popolo, was called the Gate of St. Valentine. The name seems to have been taken from a small church dedicated to the saint which was in the immediate neighborhood. Of both these St. Valentines some sort of Acta are preserved but they are of relatively late date and of no historical value. Of the third Saint Valentine, who suffered in Africa with a number of companions, nothing further is known.
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