When we speak about personal plans or arrangements for the future we use the present continuous. We knew about those plans before the moment of speaking, they are not spontaneous.
What are you doing tomorrow? ( = What are your plans for tomorrow?)
Helen is having a party on Saturday. Do you want to come? (= Helen decided to arrange a party on Saturday. Do you want to come?)
I'm not going on holiday this summer. (= I know I won't go on holiday because I've got other things to do).