Are you talking about the first pack of pills? Are you just starting the pills? If you are, you have three options:
-Start today (sunday) and you will be protected right away if you think today is the first day of your period (or the day before, that would work too)
-Wait until you're sure it's the first day of your period and you'll be protected right away
-Take it any darn day you please and you'll be protected AFTER seven days of taking the pill.
Take your pills on schedule regardless of what your period does.

If you're ever in doubt, just use backup for seven days and you'll be protected.

Georgia is good she covered about any option you could think of!

When I started my first pack of pills (coming off of the patch mid cycle) my doc said I had to use back up for a whole month. Do you know why not just seven days?

I have no idea lol. Some doctors say this- I've always had docs tell me seven days, and I tend to believe that's correct because the package inserts say the same thing. But do what your doctor says. It might've had something to do with coming off the patch.