When a female of Caucasian descent gives you oral sex the same night she meets you.
The Special is so named because US Navy sailors positioned in Europe during WW2 would often refer to females as Mers (short for Mermaids). During visits to shore to refuel and stock up on supplies sailors would often try to seduce members of the opposite sex. If a Sailor succeeded in receiving fellatio it became commonly known as the Mer Special.
I was at a party last night and got a mer special in the bathroom.