n. - A day to yourself where you stay in your metaphorical "shell" and do only what you want to do.

Typically, a person will stay in bed with a laptop and/or pen and paper. Maybe under blankets (like the turtle's shell)

Intelligence agency term for "psychological operation". A government or corporate-sponsored operation, usually taking the form of a "terrorist attack" or "crazed gunman on a spree", with the intent of panicking the public into demanding more police and laws inhibiting freedom. Psyops are usually carried out by drugging a civilian or group of civilians with aggression-promoting drugs, psyching them up, arming them, and sending them out to commit mayhem. Government-sponsored terrorism. See also blackshirts, conspiracy

A day in school where one's backpack is so ponderous it looks like a turtle shell. Such magnitude in a backpack has often resulted in falling over and having one of your slightly less turtle-y friends help you back up.

A day when after every word you say you have to say turtle.