I have heard that there is only about three days within a month that you are extremly fertile for some women. If you have a normal 28 day cycle (period starts around the same time every month) then you can most likely garentee that your fertile time which is during ovulation will be within a week and half to two weeks before your period.
I had parcticed the rythem method over birth control and both of my children were planned and they were concieved within the month after planning just by breaking the rythem.
Within 6 days before and 6 days after (and during) it is the safe zone, unless you have a real wacked ovuating cycle, but anyways the days left over within the month are most likely going to be your most fertile time.
I hope I have been of some help have a great day!!!!!