According to Wikipedia, ovulation occurs when the ovary releases a secondary cell called an ovarian oocyte, or egg, from a mature follicle. The National Institutes of Health states that ovulation occurs around the midpoint of the menstrual cycle, which is approximately 28 days long for the average woman.
According to the NIH, the ovulation cycle begins when estrogen levels fall below a certain level. When this happens the pituitary gland releases follicle-stimulating hormone, or FSH, which causes a follicle to prepare an oocyte for release near the surface of the ovary. The maturing follicle and egg then start to secrete estrogen which tells the pituitary gland to stop secreting FSH and start secreting luteinizing hormone, or LH. LH helps the follicle release the egg. The follicle breaks open, and the egg is released into the fallopian tube. The egg then travels through the fallopian tube, where it may or may not be fertilized, into the uterus. The empty follicle then secretes progesterone, which signals the uterus to prepare its lining to accept a fertilized egg. If the egg is not fertilized and does not embed itself into the uterine lining, pregnancy does not occur. The follicle stops secreting progesterone. When the uterus no longer receives progesterone, it starts to shed its lining. This shedding process is called menstruation, and the first day is considered to be day one of the 28-day cycle.