Something said by obnoxiously cheerfull bastards when someone is feeling tired and/or cranky. Mondays tend to be a horrible day since it's the first day of the working week. Made famous by the movie Office Space.
Peter Gibbons: Let me ask you something. When you come in on Monday, and you're not feelin' real well, does anyone ever say to you, "Sounds like someone has a case of the Mondays"?
Lawrence: No. No, man. Shit, no, man. I believe you'd get your ass kicked sayin' something like that, man.