Valentine's Day has its origins in the Roman festival Lupercalia, observed on February 15th. Lupercalia celebrated the coming of spring (in the Roman calendar February was observed later in the year than it is today).
The Roman gods Lupercus and Faunus were celebrated during Lupercalia. Lupercus watched over shepherds and their flocks; Faunus (like the Greek god Pan) was a god of flocks and fertility. The day became a celebration to ensure the fertility of flocks, fields and people.
On Lupercalia, the Romans sacrificed goats and dogs on Palatine Hill (fabled birthplace of Romulus and Remus). Young men called Luperci would then race in the streets beneath the hill wielding goat-skin thongs called februa. With these thongs they would lash women gathered in the streets. A februa lashing (a februatio) supposedly ensured fertility and easy child delivery.
Celebration of Lupercalia spread with the Roman Empire. The first Valentine-like cards may have been exchanged in Roman-conquered France. In a form of lottery, women placed their names (possibly accompanied by love notes) in a container. Men would seek (or were guaranteed -- this detail is obscured by time) the "favors" -- whatever those might be -- of the woman whose name they drew.
As Christianity began to take hold in Europe, the Roman church attempted to clean up this pagan day by merging its feast with Saint Valentine's Day (observed February 14). This day honored two legendary Christian martyrs.
The legend of these martyrs may have stemmed from real people or from a single person. One of these martyrs, Valentine, is believed to have been a Roman priest and physician who was killed in the 3rd century during the persecutions of the Emperor Claudius II Gothicus (the Groth). After his death he was buried in the Roman road Via Flaminia. Pope Julius I later built a basilica above his grave. A second Saint Valentine candidate is believed to be a bishop of Terni (a province in central Italy), who was executed in Rome.
These men earned their status from legends of harboring Christians from persecution, curing a blind cell-keeper's daughter and conducting marriages when they were forbidden in wartime. Perhaps this last repute and the traditions of Lupercalia coupled to honor Saint Valentine as the patron saint of lovers.
In 1969 this feast day was dropped from the Roman Church Calendar.
From Valentine's Day's association with Lupercalia and fertility comes the holiday's association with love and romance. The day's enthusiasts co-opted the Roman god Cupid as a patron of Valentine's Day. Cupid is also known as Amor and as Eros in Greek mythology. Eros supposedly impregnated a number of other gods -- Gaea (mother earth) and Chaos. The ancient Greeks believed Eros was the force of love -- a force they believed was behind all creation.
Cupid is often depicted with wings and carrying a bow and quiver of gold tipped arrows. The son of Aphrodite, Cupid is said to be mischievous and able to inspire love with a shot of one of his arrows.