Should The Contraceptive Pills Be Taken On The First Day Of Menstruation?

I just gave birth last december 2010 and I missed my period for 2 months and it come back and last for 20 days days now. What will I do? My doctor gave me a pill to take and the pharmacist said I will wait till sunday to start taking it. What will I do If I will wait to take a pill on sunday if my period stop on before sunday? Should I wait to take my pill on my next first day of period?