Saint Valentine's Day originated from a Christian martyr named Valentine who was killed on February 14th for defying the Roman Emperor Claudius II. In the 5th century, Pope Gelasius designated February 14th as a day to honor Saint Valentine. It later became a day for exchanging love messages and gifts. In the 1800s, commercial Valentine's Day cards became popular in the United States. Today, Valentine's Day is celebrated as a day of romance and love.