Monday! The day when we all go back to work. So yes, it means the unhappiness that you have to start work again after the weekend: how you feel on Monday. When we say "a case of the ****" it refers to some kind of disease or illness. Also note The Boomtown Rats' song "I Don't Like Mondays", the Bangles' "Manic Monday" and the British group with the ironic name, 'The Happy Mondays'.

We also say - a case of the Monday Blues. When you are feeling sad because you are going back to work etc...