To maximise the chances of a successful conception, you need to know a few basic facts about your wife's menstrual cycle. As others have said here, time of ovulation in the cycle is the best. If your wife's cycle lasts an average of 28 days, ovulation (which should be mid-cycle) will be on Day 14, give or take a couple of days. You therefore need to have sex, ideally daily from Day 11 to Day 17; to maximise the chances of success. Day 1 of the cycle is the day her period starts.