The first Valentine message (apart from the one St Valentine wrote himself!) is thought to be a poem from Charles, Duke of Orleans to his wife in 1415.
He was captured at the Battle of Agincourt and was imprisoned in the Tower of London to await execution.
But Valentine's Day didn't become popular in the UK until the 17th Century. By the 18th Century it was traditional for people to swap handwritten messages of affection.
Printed cards soon replaced these, making it easier for people to say "I love you" secretly.