The time when an egg is produced from your ovary is called OVULATION. This is normally in the middle of your monthly cycle (around day 14 if you have a 28 day cycle).
Your egg will live for about 12-24 hours after ovulation. If conception is to take place it must be fertilised by your partner's sperm in your body within this time. Sperm can live for several days inside your body. If you make love a day or so before ovulation, your partner's sperm will have time to travel up the fallopian tubes and will be waiting when your egg is released.
So the chances are highest if you make love on the day before ovulation.
Along with ovulation, other factors are also instrumental in ensuring a healthy conception.