Hello. I have gone through your question and understand your concern. It’s quite normal to have cycles of different days as it is a hormone regulated activity and can be affected even with stress. Ovulation day can be determined by various methods including a simple urine dip stick test similar to urine pregnancy test. Ovulation predictor kit (OPK) are ease to get and use, depending on the brand you are using urine have to be collected in a cup or hold the test stick in the mid stream of urine. Colored bands will appear on test card or stick to indicate whether or not the LH surge is occurring (LH surge is increase in hormone level before ovulation). Follow the instructions on the leaflet of the kit carefully to obtain accurate results. Intercourse throughout the cycle will be very helpful if you are trying to conceive. Good luck. Hope your concern is answered. Thank you.