Fertile days refer to the period when a mature egg is released from ovaries and ready to be fertilized by the sperm. Your fertile days occur during the ovulation period, approximately five days before the ovulation starts. In general, fertile days for women are ranging from 12 to 16 days before the next menstrual cycle. In other words, women will have their fertile days on the 10th day to the 17th day after the first day of the last menstrual period on average. This applies for women with regular menstrual cycle of 28 days. However, if you have a different cycle, you can learn and decide when your fertile days will start.