The Monday back to work after a killer party or parties. Can also be used to describe any day that just really sucks ass.
co-worker: Hey are you okay? There's a pair of woman's underwear on your back, and you have the word "place balls here" tattooed under your mouth.
you: Oh, don't worry about that. I just have a case of the Mondays.
co-worker: Thank googness. I was getting worried.