According to the Catholic Encyclopedia's entry on St. Valentine's Day the reason for having St. Valentine's on February 14th is:
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 dayFor this reason the day was looked upon as specially consecrated to lovers and as a proper occasion for writing love letters and sending lovers' tokens.
Whan every foul cometh ther to choose his mate.
It seems that Valentine was a real person. From Catholic Online's St. Valentine page:
Since he was caught marrying Christian couples and aiding any Christians who were being persecuted under Emperor Claudius in Rome [when helping them was considered a crime], Valentinus was arrested and imprisoned. Claudius took a liking to this prisoner -- until Valentinus made a strategic error: he tried to convert the Emperor -- whereupon this priest was condemned to death. He was beaten with clubs and stoned; when that didn't do it, he was beheaded outside the Flaminian Gate [circa 269].The article then goes on to state that he is the patron saint of affianced couples, bee keepers, engaged couples, epilepsy, fainting, greetings, happy marriages, love, lovers, plague, travellers, young people. He is represented in pictures with birds and roses.
It seems that St. Valentine was suffered martyrdom on February 14th and thus his name was associated with that day. Have a happy Valentine's Day.