Despite what you may have read on the Internet, St. Valentine's Day was not started by greeting card companies, candy makers or diamond sellers. The holiday's origins are rooted in the ancient Roman festival of Lupercalia, a fertility festival, which was traditionally celebrated on February 15. In 496 CE, Pope Gelasius I co-opted this pagan festival as a Christian feast day and declared February 14 to be St. Valentine's Day.
That's where the real mystery behind St. Valentine's Day begins. When he declared February 14 as St. Valentine's Day, the pope did not specify for which St. Valentine he intended the holiday. Remarkably, according to the Catholic Encyclopedia, there were at least three St. Valentines, all of whom appear to have been martyred on February 14.
In 1969, the Catholic Church revised its liturgical calendar to eliminate feast days for saints with questionable origins. As a religious feast day, St. Valentine's Day was eliminated, but the holiday remained a popular secular event. By the time the Church eliminated it as a Christian feast day, the secular Valentine's Day custom of giving handmade cards or other gifts was already well-established in both England and the United States.
Today, Valentine's Day is big business for greeting card companies, even if they did not start the holiday. According to the Greeting Card Association, 25% of all cards sent each year are Valentine's Day cards.