"Are you at the stadium Saturday?"
Translation:Sei allo stadio sabato?
"Sei tu" has a meaning of "Is that you... ?" due to the fact that Latin languages use pronouns for emphasis and don't put the verb before the pronoun when making a question.
"nello" means "in the", nor "at the", though congrats on spotting it was a "lo", I always miss them. "at the" is more general, it could be outside as well as inside the stadium
"lo stadio di sabato" could be literally translated as "the stadium of Saturday" or "Saturday's stadium" implying that there is a different stadium for some of the other days of the week :P