I haven't read every reponse just yet, but merely responding to the OP's question.
I think in all my hobby days I've only ever cancelled one appointment. It was days in advance, coming only because I'd miscalculated how much disposable income I'd have available to play with. It was the only reason why. I wasn't intercepted with a sweeter deal or any other [phoney] reason why people cancel.
In this situation (the day of) I wouldn't have cancelled, even if it meant spending money I didn't want to spend. It'd have to be a life decision at this point for me to cancel, something like, if I hobby today I can't pay my rent tomorrow. The day of the event, I'm pretty much locked in, outside of freak circumstances like death, car accident (beyond a 'love tap', where somebody will have to shell out $$) or sudden illness. I've had that before, too, where I wake up feeling fine but say 2 hours later I'm crawling to a doctor.
It's just not in my character to cancel on someone for reasons that aren't superficial. I mean, even if my ATF called and said come over for 2 free hours of 'do what you want to me', I'd still go through with the appointment.
Naah, I might have to cancel for that, but I'd offer a makeup session plus a huge tip.