How do you know when to take the pill?

So I'm not on birth control yet but I have an appointment next week for it. I use tampons when I'm on my period and sometimes I don't exactly know when my period ends because I get a lot of woo it's over for it not to be over. So how do you know when to switch from the placebo pill to the actual pill? And if are off by a day either way, what can happen?