The origins of Valentine's Day are shrouded in mystery. According to University of Notre Dame Professor Lawrence Cunningham, scholars have two main theories to explain how February 14 became synonymous with romance:

• Roman Feast of Lupercalia - This ancient pagan fertility celebration, which honored Juno, queen of the Roman gods and goddesses and goddess of women and marriage, was held on February 14, the day before the feast began. During festival time, women would write love letters, also known as billets, and leave them in a large urn. The men of Rome would then draw a note from the urn and ardently pursue the woman who wrote the message they had chosen. (Apparently, the custom of lottery drawings to select valentines continued into the 18th century, coming to an end when people decided they'd rather choose -- sight seen! -- their valentines.)
• The Birds and the Bees? - In the Middle Ages, people began to send love letters on Valentine's Day. Medieval Europeans believed that birds began to mate on February 14.

There's also some controversy regarding Saint Valentine , for whom the famous day is named. Archaeologists, who unearthed a Roman catacomb and an ancient church dedicated to St. Valentine, are not sure if there was one Valentine or more. Today, the Catholic Church recognizes at least three different saints named Valentine or Valentinus, all of whom were martyred on February 14 -- at least two of those in Italy during the 3rd century. The most popular candidate for St. Valentine was a 3rd century Roman Pries who practiced Christianity and performed secret marriages against direct orders from Emperor Claudius II , who believed single soldiers were more likely to join his army. Legend has it that Valentine sent a friend (the jailer's daughter) a note signed "From Your Valentine" before he was executed on February 14 in 270 A.D. (That phrase is still used prominently on today's cards!)

Early Christians were happier with the idea of a holiday honoring the saint of romantic causes than with one recognizing a pagan festival. In 496 A.D., Pope Gelasius named February 14 in honor of St. Valentine as the patron saint of lovers. In 1969, Pope Paul VI dropped it from the calendar. However, the blend of Roman festival and Christian martyrdom had caught on, and Valentine's Day was here to stay.

Source: BBC