If you want to know when you're most fertile, you need to know when you ovulate. Start counting beginning on the day you start your period. The day you start is day one. Around day 14 of a 28-day cycle, you will probably ovulate. In other words, it's the time you're most fertile. If your cycle is more than 28 days, you'll need to count accordingly. During ovulation, your body releases an egg that's viable for about 24 hours. Sperm is viable for 48 hours and must make contact with the egg during this time. Most, but not all, women ovulate on the 14th day. However, remember that this isn't true of all women each month. Even if you ovulate on the 14th day one month, it doesn't mean you'll ovulate on the 14th day the next. Your cycle can vary from month to month. Some women ovulate a little earlier, others a little later. Some women don't ovulate at all. You can still get your period even if you're not ovulating.