The St. Valentine's Day Massacre
No Valentine's Day would be complete without a reference to one of the most notorious crimes of the century: the 1929 St. Valentine's Day Massacre that took place in a warehouse/garage on the north side of Chicago. A good summary of it can be found here.
A good movie for celebrating this notorious holiday with your significant other would be this one:
Some Like It Hot is one of the greatest commedies in the history of American cinema, thanks to the three principle actors: Jack Lemmon, Tony Curtis, and Marilyn Monroe. Plenty of romance for the romantic types, and lots of commedy for those not too concerned about the romance. And the first "day" of this movie is Valentine's Day!
But the reason I picked this film is due to the opening sequence where Tony Curtis and Jack Lemmon accidentally witness one of the most notorious crimes of the century: the 1929 St. Valentine's Day Massacre.