It may not be fair to say that Valentine's Day has its origin in antiquity, but there were romantic spring holidays (Gamelion and Lupercalia) and a St. Valentine or two.
Valentine's Day Saint #1: There may have been a real Valentine, a third-century priest who defied Emperor Claudius II's ban against wartime marriages. According to legend, Valentine performed secret marriages until he was discovered, put to death, and buried on the Flaminian Way.
See "St. Valentine, Chaucer, and Spring in February," by Jack B. Oruch Speculum, Vol. 56, No. 3. (Jul., 1981), pp. 534-565, for why this doesn't work historically.
Valentine's Day Saint #2: There's another legend in which a Valentine, persecuted for helping Christians, restored the eyesight of his jailer's blind daughter, and then maintained a secret correspondence with her to which he signed his name "your Valentine."
Another Derivation of Valentine: Even more speculative is the notion that Valentine's name was originally "Galantine," signifying "gallant," a word with more obvious associations with courtship. The shift in consonant to "v" is explained as the way medieval French peasants pronounced the letter "g."
Valentine was a popular name among the Romans, with emperors named Valens and Valentinian.
Christianization of Lupercalia: Another theory is that Pope Gelasius I replaced the pagan festival of Lupercalia with the Christian Feast of the Purification, which was celebrated on February 14, 40 days after Epiphany. This is based on Bede who wrote about pagan customs in February, and not specifically Lupercalia. Oruch says it wasn't until the 16th century that the pagan ceremony of Lupercalia was said to be behind Candlemas (February 2).
February's Special Holidays: Imbolc, Oimelc, Brigit's Day, The Feast of the Purification of the Virgin Mary, Ground Hog's Day, and Candlemas are all holidays that occur in the first half of February. Some believe the Christian holidays are simply renamed pagan ones.
