I think to some extent it's less an absolute question of correctness and more an issue of trying to say things naturally, as a native speaker would.
For example, everyone will understand what you mean if you say, "I will go to a party Saturday," but under normal circumstances a native speaker would say, "I'm going to a party Saturday."
Also, it's confusing, as under some circumstances, "I will go to a party Saturday" would be the natural choice.