From the Joyofbaking.com
February 14th is named after the patron saint, St. Valentine, and we celebrate this day with the exchange of candy, flowers, cards, and gifts as a token of affection to our loved ones. The history of this day is very sketchy but it does appear to derive from Christian and Roman traditions. The story I like dates from the third century when Rome was ruled by the Emperor Claudius II. The Emperor outlawed marriages for young men as he felt single men made better soldiers than men who were married.
A priest, named St. Valentine, didn't agree with the Emperor and married young lovers in secret. When the Emperor discovered what St. Valentine was doing, he sentenced him to death. While St. Valentine was in prison, waiting to be put to death, he met and fell in love with the jailor's daughter. Before he died he sent her a love letter and signed it "From your Valentine".