Mondays are the worst days of the week for Garfield. On Mondays, it is common for Garfield to be pied in the face or some other bad thing to happen.
Often, Mondays are off-panel but shown causing things to happen in-panel (such as throwing a pie at Garfield). When they are shown, they are drawn as ugly monsters. The most prominent theme was "The Monday That Wouldn't Die", in which every day in the week after a certain Monday was also Monday.