I don't exactly understand his reply, "On Opposite Day it is.". Is he saying "Opposite Day is beautiful." or "The mess looks beautiful on Opposite Day."?

Native speaker’s answer
Rebecca
He's saying that "the mess looks beautiful on Opposite Day." Implying that when someone says that it looks beautiful on Opposite Day, they actually mean: it looks terrible! So he's still saying it's a mess and looks bad. I'd say this is a joke mostly between kids. When you say that it's Opposite Day, everything you say means the opposite. Ex: I start my diet on Opposite Day. Ex: It's Opposite Day! I want to do all my homework today.