Originally the feast day of Saint Valentinus, who was punished for performing marriage ceremonies between Roman soldiers and Christians. Popular media (advertising) has turned it into a day to spend money on your sweetheart. Ain't that romantic?
As with most Christian holidays, valentines day has pagan origins. Lupercaila was celebrated on February 15 by pagans for many years before the church hijacked it. Lupercalia was a fertility festival dedicated to Faunus, the Roman god of agriculture, as well as to the Roman founders Romulus and Remus.