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I believe that the commonly accepted answer is that our mythology of St. Valentine is first recorded in Chaucer.
The popular customs associated with Saint Valentine's Day undoubtedly had their origin in a conventional belief generally received in England and France during the Middle Ages, that on 14 February, i.e. half way through the second month of the year, the birds began to pair. Thus in Chaucer's Parliament of Foules we read:
For this was sent on Seynt Valentyne's day Whan every foul cometh ther to choose his mate.
The cited article contains additional information, including the fact that of the three Valentines who are Saints, none of them have any particular association with marriage, and some additional information about historical growth of the holiday.