Why Am I Getting Irregular Menstrual Periods?

last three my periods are delayed and when take regestrone for four days and after the completion of the course the fourth day i get my periods e.g my last period was on 21st feb but in mar i did not got that on 21st so i waited till 23rd and 24th i started taking regestrone and on 30th mar i got my periods what must i do for regular periods