You are half right. The question is not to do with whether you are sure or not, but about whether this has been planned with OTHER PEOPLE involved. When we use a present continuous to talk about a future event, it means that it is a fixed ARRANGEMENT, involving other people. If you say 'I'm playing football tomorrow', this means that you have arranged this with others - your name is down on the schedule, and the other team members are expecting you to play. Or if it's an informal game, you have arranged a time and a place to play football with your friends. 'I will play football' is about what you personally intend to do. Maybe it is your plan. Maybe you're 100% certain, but no arrangement has been made with other people. That is the key difference. As for what the other person told you about playing all day - that is completely wrong. Nonsense. Rubbish. Never listen to another word they tell you about English!

Yes, you are correct. The 2nd sentence is a planned event. The 1st sentence, maybe I will and maybe I won't. No. That person was incorrect. Usually we indicate if that event will take place all day within that sentence or through the context of the conversation. The ending -ing does not indicate you will do the event/activity all day long.