This was a Roman fertility festival that was already ancient by the time of the Empire, and continued on for some time after the Empire had been Christianized. I believe it wasn't until the late fifth century that it was officially abolished, although it had changed considerably from its original form by then. Shakespeare's Julius Caesar begins on the Lupercalia. I don't believe any official connection with Valentine's Day has been found, but the fact that it was celebrated at the same time of the year (actually the thirteenth through the fifteenth, according to Wikipedia) makes for an interesting coincidence, if nothing else.
2. St. Valentine
There were apparently several early Christian martyrs named Valentinus, and all that's really known about this particular one is that he died on the fourteenth of February. Legends have grown up about how he performed Christian marriages at a time when this was illegal, but I don't believe there's any actual confirmation of this, and it might have been invented to fit with the later association of St. Valentine with lovers. This connection apparently wasn't established until the fourteenth or fifteenth century, and Valentine isn't on the Catholic Church's official list of saints.
Cupid becoming a part of Valentine's Day also seems to be a relatively modern idea. The god associated with the Lupercalia was not Cupid, but Lupercus, often identified with the Greek Pan. If that association had kept up, it certainly would have resulted in quite different cards.
But the association of the day with lovers means that the current mythological symbol is the god of love. Cupid (or Eros in Greek) is an interesting figure, described by Hesiod as being one of the first gods to emerge from the primordial Chaos, but later retconned to be the son of Aphrodite. Mind you, his possible mother had different origins of her own, sometimes said to have been born when Ouranos' testicles were cast into the sea, and other times to be the daughter of Zeus and Dione. I think they might need some paternity tests up in Olympus. ("Castrated testicles of the primordial sky god, you are NOT the father.") Anyway, the classical portrayal of Cupid seems to have been as a naked boy:
More recent conceptions often give him a diaper or a toga, and make him look even younger, hence presumably unable to participate in any of the love he's always forcing upon others. One of the most famous stories about him, however, is that of his marriage to Psyche, presumably meaning that some myth-makers thought of him as somewhat older.