Sorry, but I cannot go on the picnic with you. I'm shopping for clothes with my elder sister on Saturday.
It means that 1 have already planned it for Saturday, my sister knows about it and is expecting me to go with her.
I'm taking my exam on Monday. I feel so nervous.
It means that taking the exam is in my plans for a certain time on Monday.
I have this weekend absolutely free. I think I'll shop for clothes with my elder sister on Saturday.
It means that this is not my plan but the present moment decision. My sister doesn't know anything about it yet. and she may refuse to take me with her.
I don't worry at all. I'm sure I will pass it. I know the subject perfectly.
It means that I can't plan it, but / think it will happen in the future.