I know is sort of thing can drive you bonkers. The only way i found to cope with my FIL asking the same question over and over (I once counted 35 times in one hour) was to make a sort of separate compartment in my head, where I could go on answering nicely, but mechanically. I am not the most patient person in the world, but I did manage it somehow.
It'll be a bit late now for the hair appointment, but many people find that telling people well in advance about any appointment or event, only gets them in a fuss, or agitated, and endlessly fretting and asking about it, as you have found. So the advice is usually, don't tell them until the day, or maybe the night before - saves aggro all round.