A corruption of galantin (a lover, a dangler), a gallant. St. Valentine was selected for the sweet-hearts' saint because of his name. Similar changes are seen in gallant and valiant.
Valentine. One of the Two Gentlemen of Verona; his serving-man is Speed. The other gentleman is Proteus, whose serving-man is Launce. (Shakespeare: Two Gentlemen of Verona.
Valentine, in Congreve's Love for Love. Betterton's great character.
Valentine (The Brave). Brother of Orson and the son of Bellisant, sister of King Pepin and wife of Alexander, Emperor of Constantinople. The twin brothers were born in a wood, near Orleans, and while their mother went in search of Orson, who had been carried off by a bear, Pepin happened to see Valentine and took him under his charge. He married Clerimond, niece of the Green Knight. (Valentine and Orson.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894