The rhythm method is based on the premise that ovulation occurs approximately in the middle of the menstrual cycle. Typically, ovulation happens around 14 days before the start of the next menstrual period. To use this method, a woman needs to keep track of her menstrual cycles for at least six months to establish a pattern. She then calculates her fertile window by identifying the longest and shortest cycles and using a formula to determine the days she is most likely to be fertile.