Both tenses are acceptable but there is a slight change in emphasis and perspective of the event.
To use the Present Simple for a future event means the event is tied to the present (and past). In what way? Well perhaps you have tickets, or it is on a website etc etc. In some way there is a connection and importance to including it in the present.
To use Will emphasizes that action is 'not before' in a way. It isolates it in that future moment. What does that mean!? Let's look at a couple of examples:
I am on holiday next week (so it is a future fixed plan) so let's finish the project this week. (I am interested in the present time).
I will be on holiday next week (so a future 'fact') so don't call me! (I am interested in what happens next week.)
Or another example.
The election is next week, what are you planning to do on Election Day? (emphasis is on the present)
The election will be next week, I hope it turns out ok! (Emphasis is on the future)
So directly with your answer....hmmm... I believe that you want to emphasize Saturday. You are NOT discussing the crazy preparations happening now! But you are jumping to Saturday. I think in this case WILL is 'possibly' preferable.
It is one of my favourite topics (the meaning of tenses) I highly recommend The English Verb by Michael Lewis and if you want I have some interactive posts on the subject here The Meaning of the Tenses