Probable ova releasing day is $12-14$ day of menstrual phase, and sperm can live for $2-3$ days in female reproductive system. So, $10-17$ days are the most probable days for fertilisation.

Draw backs of periodic abstinence The effectiveness of this method is limited because only a few women have regular menstrual cycle and the actual time of ovulation can not be produced as the ovulation in humans occurs about $14$ days before the onset of the next menstruation