There are different methods of knowing ovulation day.calender method ,kit for ovulation and usg evidence of ovulation which is the most appropriate one.luteal phase is mostly constant which is 14 days.so from her last period ,calculate her tentative next cycle and minus 14 days from the expected date.then u can add and substract 3-4 days to find a week which is the most probable week for pregnancy

You should go for folliculer study (ultrasound examination of folliculer growth ) to check yr exact day of ovulation.otherwise in reguler cycle ovulation occur 14 days befor period date

Do follicular study or use ovulation test kit

In irregular cycles its difficult to calculate ovulation day U can use home based LH kit or undergo follicle study on Sonography to see approximate dates of ovulation