A substitution for the well known Casual Friday (where you're allowed to dress down in your office on Fridays) that instead allows you and your co-workers to drink irresponsibly until someone needs medical attention. Everyone else is allowed to go home early when the ambulance arrives.
Xavier: The new boss just instituted Casualty Fridays!
Ally: Call the ambulance now; Alice won't make it past her third drink.
Ally: Call the ambulance now; Alice won't make it past her third drink.
by Capital X May 4, 2012