Advice quick!

Hey I'm looking for some advice, my periods were due yesterday 8/12/15. They haven't arrived I am usual very regular, 28 day cycle. I took a test yesterday and it said it was negative. I have a tattoo booked on Friday which has been booked for months for my birthday. Im really scared my periods still don't arrive but tests still say negative. I am going to take another test tomorrow morning to see what it says but was wondering what people think I should do if my periods don't come by Friday but test still say negative? I don't want to cancel the appointment and then find out I'm not really pregnant :/