Why have you booked to get a tattoo before you've settled on a design? Or are you hoping to just turn up and have it done there and then? Coz if the tattoo parlour do that they're breaking the law, and when you're trusting someone to stick a needle in you you need to know they're playing by the rules...
Because before any kind of unnecessary surgical procedure is carried out there must be a period of time (usually only a week or two) given in which the person can change their mind.
The main issue though is that a tattoo is something that will be with you for the rest of your life (until you're old and wrinkly and weeing in a bag) so it has to be something you really love, otherwise you'll only regret it later.
I also think you should be 100% sure what you want and where you want it before you even think about booking an appointment, however many you have already. Or are you just getting another one for the sake of it?
As I said its a friend who's doing it and I can cancel anytime I like. If I don't see the right design I won't get it done. Its something I've wanted for a long time and If i see the right design I will know, and if I don't I won't have it done - its as simple as that really.