Monday is a recurring theme throughout the Garfield comics. Monday is an entity that, logically, appears on Mondays, where it harasses Garfield through a combination of bad luck and practical jokes.
The majority of times, Monday stays off-panel and throws pies at Garfield or manifests itself as an unrealistically large amount of misfortune. An example of this is when the ground below Garfield suddenly breaks open, causing him to fall in a hole. During the times Monday appears on-panel, it usually is portrayed as a hideous monster.
A prominent theme was "The Monday That Wouldn't Die", in which every day in the week after a certain Monday was also Monday.