Saint Valentine risked the Emperor's wrath by standing up for traditional marriage, secretly marrying soldiers to their young brides.
Condemned to die for refusing to worship pagan idols, Valentine prayed for the blind daughter of his jailer, resulting in her healing, the conversion of the jailer and many others. Right before his execution, Saint Valentine wrote a note to the jailer's daughter, signing it, "from your Valentine." February 14th marks the date of Valentine's martyrdom.
Source: American Minute with Bill Federer

The Story of Valentine's Day from The History Channel. According to a variety of resources, the legends and stories about St. Valentine are based on the execution of three different men, named Valentine, who lived in the Third Century. Two of the Valentines made the mistake of drawing the attention of Emperor Claudius II, who had been persecuting the Christians throughout the Roman Empire. One of them is said to have been the Bishop of Terni, Italy. They stood up to the Roman authorities and held to their beliefs and in different locations, on different years of Claudius’ reign, were executed on February 14th, and buried by the side of the Via Flaminia, the road between Terni and Rome. Two centuries later, when Valentine was granted sainthood, Pope Gelasius chose February 14th as Valentines’ Day in an attempt to replace Lupercalia with a Christian celebration. The third Valentine is barely mentioned by the ancient stories as having been murdered by a mob somewhere in Africa. Click "more" for source citations.

Image credit: history.com
Who was St. Valentine, and why do we celebrate him on February 14? In honor of Valentine’s Day, get the facts about this enigmatic character. (article by Elizabeth Hanes)

The origin of St. Valentine's Day was certainly not a story of flowers, chocolates, and songs. Read as David Kithcart explains why Valentine's Day did not initially start as the romantic holiday it now has become.

The first printed mention of Valentine was in the Nuremburg Chronicle, 1493. Image citation: Valentinus. In Liber Chronicarum, compiled by Hartmann Schedel. Germany: A. Koberger, July, 1493. World Digital Library.

Cards and gifts were exchange beginning in the 1500’s and machine made cards came into popularity in the 1800’s. Etymology of “valentine” as “a letter or card sent to a sweetheart” began about 1824, is in common use today, and can be found in every dictionary.

Who Prays to St. Valentine? by Alma R. Giordan. This light-hearted article mentions other parts of the legend. The birds and leaves that Hallmark puts on their cards come from the legend of the birds that carried messages, on leaves, to Valentine’s heartbroken followers. The author’s opinion is that Hallmark is among those who pray to St. Valentine, as well as, florists and candy makers, even John Donne.