What does blue Monday mean?

well... I know it's a song by New Order... but I have no idea where it came from

Though it's probably a mostly meaningless title, I would guess it's a reference to the let down you feel after a long weekend of too much partying. New Order's inspiration at this point were NYC clubs, where Ecstasy was beginning to flourish in its early days, and I've also heard it said that the Monday following a rave was sometimes called a "suicide Monday" by the British.