From "Clinical Gynecologic Endocrinology and Infertility", sixth edition, Leon Speroff, M.D., etc. This is THE medical textbook for infertility. Pages 226 - 227.

"The preovulatory follicle, through the elaboration of estradiol, provides its own ovulatory stimulus. Considerable variation in timing exists from cycle to cycle, even in the same woman. A reasonable and accurate estimate places ovulation approximately 10 - 12 hours after the LH peak and 24 - 36 hours after peak estradiol levels are attained. The onset of the LH surge appears to be the most reliable indicator of impending ovulation, occurring 34 -36 hours prior to follicle rupture. A threshold of LH concentration must be maintained for 14 - 27 hours in order for full maturation of the oocyte to occur.  Usually the LH surge lasts 48 - 50 hours.

Because of the careful timing involved in ivf programs, we have available some interesting data. The LH surge tends to occur at approximately 3AM, beginning between midnight and 8AM in over two-thirds of women. Ovulation occurs primarily in the morning during Spring, and primarily in the evening during Autumn and Winter. From July to February in the Northern Hemisphere, about 90% of women ovulate between 4 and 7PM ; during Spring, 50% of women ovulate between midnight and 11AM."