A womens menstrual cycle is on average 28 days long. Hormone release occurs due to signals from the brain. Days 1-5 is when a period occurs and levels of all hormones are low. After this, LH (luteinizing hormone) and FSH (follicle stimulating hormone) start to rise, peaking at day 14 and then dropping quickly and staying low till day 28.
This rise in LH and FSH causes oestrogen to increase, triggering ovulation. Oestrogen has a second smaller peak around day 21.
Also peaking at around day 21 is progesterone. It then decreases until eventually levels are low enough to causes menses.