"What? What's that," He asks slowly. Gon blinks. "What's what, Killua?" Killua is suddenly shy, is this something he should know about? "Um, you know, uh," Killua struggles, "Birth day? Is that like, the day I was born? Is that something you're supposed to celebrate?" "Killua... Killua's never celebrated it before?" ... Killua doesn't know what a bday is. Gon shows him that July 7th is a happy day, not what his brother enforced on him.