While the history of Valentine's Day is sometimes debated, it clearly links back to a Catholic saint named St. Valentine.
The problem is there are actually three St. Valentine's -- one a priest, one a bishop, and little is known about the third. All were martyrs.
In 469 A.D., Pope Gelasius declared Feb. 14 a day to honor St. Valentine, one of these three men.
One legend says that a Roman emperor banned soldiers from marrying in the third century, but St. Valentine took issue with this. He became an advocate for soldiers and was executed as a result of his outspokenness.
Another legend says St. Valentine was executed for his beliefs in Christianity and just before he died, he left a farewell note for a loved one and signed it "From Your Valentine."
A conventional and widely accepted belief about the holiday itself is that Valentine's Day grew out of a Middle Ages tradition of celebrating Feb. 14 as the day "the birds began to pair."
History.com notes that February has long been associated with being a month of love, and Feb. 15 was celebrated in ancient times as a fertility festival.
Whatever its origin, it took off, and the U.S. Greeting Cards Association estimates Valentine's Day is the second-most popular card-giving day of the year, only to Christmas.