Worst day of the week is Tuesday. Official.
Monday: well, at least you've just had the weekend. There may even be a bit of an after-glow. Wednesday: half-way through. Thursday: yay, it's Friday tomorrow. Friday, Saturday and Sunday: don't really need to explain this one.
But Tuesday? Nothing. No solace at all. Named after Teutonic god of war, Tew. You only name a day after the war god if you're expecting it to be rubbish.