Of course you don't say that when you let them know you won't be in. You do the old "My stomach is jacked up I won't be into work today".

Its good to sometimes just chill on that day, or go out and do something cool on the hours you'd normally be at work. It's refreshing as hell to have that middle of the work week day off.

Matter of fact, I may take me one off next week just because.