St. Valentine lived in the 3rd century, an era of persecution under emperor Claudius II (Claudius Gothicus). This alone is reason to celebrate him. But there’s also a legend that holds that St. Valentine of Rome performed secret weddings, against the orders of the law, in a time when he wasn’t allowed to do so (and when unmarried men were considered better soldiers, dying more willingly for Caesar).
When we celebrate St. Valentine’s day, we’re not celebrating his beheading (which occurred on February 14, 278) but his saintliness, his bravery, and specifically the lovely legend of his act of marrying folks secretly. Really, all of these are different ways of expressing love… love of God, love of people, and love of the world the way it really should be. Now, aren’t you glad you know that?