This is the day when you clean the litterbox. If more than one person lives in the abode, they generally take turns changing the kitty litter. It can be an unpleasant experience; that's why they make littermaid and other self-cleaning literboxes.

Cats are such amazingly clean animals, they never smell bad (unless they are sick) and clean themselves voraciously. It's kind of ironic that their shit smells so damn bad...