You should start on the first day of your period OR the first Sunday after your period has started. That is what is says in the directions. As well all prescription meds, you should follow the directions.
If you start on any random day, it will not be in sync with your natural period cycle and you'll have to deal with your body being out of whack until it learns a new schedule.
So it doesn't matter if I'm on my period?
you can start any time or day you want long as its the sametime everyday.