"What are you going to do next Saturday?"
Translation:¿Qué vas a hacer el próximo sábado?
Days of the week, especially when it concerns something happening on a specific day or days, need the article el or los for plural. Here's good information about using the definite articles in Spanish. Scroll down to where it mentions about days of the week: https://www.thoughtco.com/how-to-use-definite-articles-3079100.