Hi.. The only purpose of stating the birth pill on day one of the menstrual period is to avoid disturbing the regularity of the period.. To explain, if you are going to start the pills before you periods are, you may not get the periods on the expected day as you are on the pills. Similarly you would be finishing the pills before the expected days which would induce the withdrawl bleed..
If you want to avoid these irregularities, it is better that you start the pills on the first day of periods. You may be on other mode of contraception till then, if you cannot avoid sexual relationship before the start of the pills...