A US law, dating back to the early 70s, that allows the observance of holidays to occur on a Monday, rather than on its official day - thus giving you the tidy 50%-longer weekend. Is it 50% better? That's up to you. Some holidays are exempt from this - we don't celebrate New Year on a January 3 Monday, for instance, but financial markets will sometimes independently take a Friday or Monday off, in order to fit such-and-such holiday in. Your useless fact of the day. You owe me a beer.