"Two Valentines are listed in the Roman Martyrology on 14 February: one a Roman priest martyred on the Flaminian Way, supposedly under Claudius, the other a bishop of Terni who was martyred at Rome, but whose relics were translated to Terni. The Acts of both are unreliable (..). Neither of them seems to have any clear connection with lovers or courting couples. The reason for this famous patronage is that birds are supposed to pair on 14 February, a belief at least as old as Chaucer, just as the custom of choosing and calling oneself a Valentine is at least as old as the Paston letters."
Source: The Oxford Dictionary of Saints (1978,1992)