Menstrual cycle
Ovulation happens if there is both a rise in estrogen and a peak in LH at the end of the follicular stage.  The rising levels of LH make the ovum mature.  The follicle that contains the ovum swells and breaks open.  (In the next stage, the leftover cells of the follicle turn into an important structure called the corpus luteum.)  The ovum is released into the Fallopian tube.