The Crab Festival is an annual celebration on Miami. With most of it's contestants been Cuban, the festival is organized at night, it is divided into tables, each with a dish in which participants must store the crabs they smash. The whole point of the festival is that the contestants can only eat what they manage to crush.
Rita invites Dexter to the Crab Festival, and both participate on the celebration. Rita actually enjoys smashing crabs while Dexter thinks to himself that the people of Miami are not so different from him (in that "normal people are so violent"). However, his date with Rita is interrupted when he notices a crime scene in the vicinity of the festival and heads towards it.