I'll see my friends on Saturday.

Question:

Is 'will' correct in this situation?

A: Have you got any plans this weekend?

B: Yeah, I'll have dinner with some friends on Saturday.

Answer:

Nope!

Why?

Have they already thought about it? Yes.

Have they planned it with someone else? Yes.

So, 'will' isn't appropriate here.

Here you can either say 'I'm going to have dinner' or just 'I'm having dinner'.