A day generally created for the purpose of making people wish they were someone else. The day you realize you have 4 days of work ahead of you and that they won't be going by fast at all. Symptoms generally include feeling like crap, wishing you were dead, or not showing up for work in general.
"Sounds like somebody's got a case of The Mondays." ~ Office Space
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.