If you call the first day of a woman's period "day one", in a textbook example of a monthly cycle, the woman will ovulate on day 14, the egg will make its way down the fallopian tubes where the womb is thickening its lining, and if it is not fertilised it will be shed, along with the womb lining as the next period, starting on day 29, which is day 1 of the next cycle. In order for fertisation to take place a sperm must meet the egg at the right place and the right time, so yes, in theory there is only a small window of opportunity for fertilisation to take place, and this window will be around day 14 -16. However, all women have different cycles to some extent, and cycles may vary from one month to another in the same woman, so you cannot be definite about the most fertile days in the cycle. Many factors can affect ovulation, and sperm can live for several days in the vagina, so relying on this method is advisable only if a)there is no alternative for ideological reasons, or b) pregnancy would not be a disaster. There are kits available which measure temperature and other factors, and used over a sufficient period of time to build up a pattern, they are a more scientific method of measuring fertility.