A 24-hour period in which you indulge yourself in complete gluttony, narcissism and sloth, in the manner of Eric Cartman on Comedy Central's South Park.
Guy1: Did you just pound down that entire box of Krispy Kremes?
Dude: Yep, I'm having a Cartman Day. A nap, then a tub of Ben and Jerrys is next, right after I get you to RESPECT MY AUTHORITAH!
Guy2: But... you're already a fatass.
Dude: Screw you guys -- I'm going home!