The Romans, not being familiar with Massachusetts weather, thought that spring began on February 5th. So, like most fertility rites, Lupercalia takes place in early spring, February 15th.
Young men would start out in the cave of Romulus and Remus, and sacrifice a goat there. Some sources (especially one named Alex) do say dog, but I think a goat would be a bit more appropiate since goats are frequently tied with sex by the Greeks and Romans. *cough, cough* Satyrs. Also because the word for goat-skin is “februum”, another word for Lupercalia is “dies februata”, and then the month in which the holiday occurred became Februarius, and thus our February.
After the required religious ritual and religious drinking (it is a Roman holiday after all) the young men would dress up in the bloody skins and run through the streets, hitting women with strips of goatskin. Young wives would try to get whipped because it brought fertility and could also ease childbirth pains for pregnant women.
This is alluded to the beginning of Shakespeare’s Julius Caesar when he tells Calpurnia to try to get hit, but actually the Lupercalia was not terribly popular during Caesar’s reign. Suetonius writes that Augustus brought it back into popularity (because it suited the moral reforms he was trying to make with the Lex Julia).
Our Valentine’s Day just celebrates those who are coupled, but the Romans being cheerfully polytheistic, used this ceremony to honor Romulus and Remus, also Lupercus, Faunus and a little-known fellow called Februus. Poor Februus was the Etruscan god of the underworld, who somehow got himself associated with sex and flogging. That’s what happens when your followers praise you in a language isolate instead of good ol’ Indo-European. Like the rest of the underworld gods, he was associated with caves, so he was probably hanging out w/ the shades of Romulus and Remus when the drunken teenagers showed up for the sacrifice and orgy.