"Ginger" is another word for a redhead. And "daywalker" comes from an episode of South Park in which Cartman gave a presentation in school about "gingers." He warned his classmates that gingers have no souls, and that because of their extra light skin, they need to avoid sunlight. When Kyle said he had red hair but didn't need to avoid sunlight, Cartman explained that some gingers are fortunate enough to be able to tolerate excessive sunlight. He called them "daywalkers." He also said that daywalkers don't have light skin and freckles, only red hair.
This never really happened, but have you seen "Funeral For a Fiend"? It was the one where Bob made a commercial about a restaurant to lure the Simpsons in, then he pretended to have a heart attack in court and die. The Simpsons went to his funeral. This is the only episode Robert and Judith are in.