The adult menstrual cycle lasts 28-35 days on average. The follicular phase is the phase before ovulation, and the luteal phase is the phase after ovulation. The follicular phase often varies between 14-21 days; the luteal phase is normally consistent around 14 days.
The follicular phase is the phase before the day of ovulation. Taking into account the average lifespan of sperm and egg (sperm live for 3-5 days, eggs for less than a day), the fertile window is 12-17 days before the next menstruation.