Way back in ancient Rome, Emperor Claudius thought that single men made better soldiers since they didn't have families to think about back home. He outlawed marriage for young men, and so Saint Valentine basically said “screw that” and married them anyway. For this crime, he was tortured and put to death.
According to Wikipedia, there were actually three Valentines who were associated with a death around February 14th. There are also links between the Roman holiday of Lupercalia, celerated February 15th. This was a fertility festival that involved the sacrifice of goats and sometimes dogs. Men would run naked through the streets striking women who wanted to get pregnant or have a good delivery with bloody strips of the flayed goatskin, called februum, which is where we get the word February.
Modern customs link the day with romantic love, but other traditions associate February 14th with the advent of spring. It is said to be the day when plants and flowers start to grow and work in the vineyards begins. Ystävänpäivä is the Finnish name for St. Valentine’s day. It means “Friendship Day” in the Finnish language, or “Day of the friends”.