Sweethearts who want to keep their relationships running strong know that Valentine's Day is celebrated each year on February 14. But even the most ardent Valentine's Day enthusiast might not know just why this day designed for lovers to express their affections for one another is celebrated in mid-February. According to the Library of Congress, it's hard to pinpoint exactly why Valentine's Day is celebrated on February 14, though the date might have ties to the ancient Roman celebration of Lupercalia. Lupercalia was a spring festival celebrated each year on February 15. The holiday was moved to February 14 after the spread of Christianity. The Christian faith had several early martyrs named Valentine, and each of them were celebrated with a saint day on February 14. But the unique history of Valentine's Day and its association with February 14 as well as its romantic sentiments does not end there. The Library of Congress also notes that, in the Middle Ages, people believed birds selected their mates on February 14. As a result, it was not uncommon for lovers to recite prose to one another on this date.