Dear thanks for choosing health care magic.Normally the menstrual cycle is of 28 days.and in most of females ovulation occurs in mid cycle.Means around day 14.So by this theory the 3 days before and 3 days after the date of ovulation the period is considered unsafe.But my dear there are no fixed rules,this is observed in most of the females .As far as prevention of pregnancy is concerned this unreliable as a day here and there can lend you in trouble.please refrain from this.Try to use other methods after talking to your doctor.