There's nothing more depressing than being told that today is the most miserable day of the year. If you're a cheery, glass half full, Pollyanna type, then you'll probably subscribe to the notion that a chirpy attitude and a big smile goes a long way to making you feel more positive about most things. So it stands to reason that being fed a notion of a particularly miserable day, is likely to have just the opposite effect.
So I take your grim 'Blue Monday' and raise you 'Bloom Monday' instead.
Isn't that better?