“How do I calculate my ovulation day?”

In a typical 28 day cycle your ovulation day is mid cycled - day 14. You can also buy ovulation kit in the pharmacy to help you determine that.

It makes a difference if you are trying to find ovulation to achieve a pregnancy or trying to avoid ovulation to avoid pregnancy. If you have a regular 28 day cycle then ovulation will occur between day 12 and 16, If you have a longer cycle you can approximate the day of ovulation by counting back 14 days from your anticipated next menstrual period. This is how the phone applications work by approximating the length of your average menstrual cycle and counting back 14 days. If you are trying to achieve a pregnancy and want to pinpoint ovulation you could use an ovulation predictor kit that will signal you about 24 to 36 hours before your will ovulate. Alternatively, you could just have intercourse around that time. If you are trying to avoid pregnancy with the rhythm method you would need to delay intercourse until at least 5 or 6 days after you think you have ovulated based on the above estimated times.

With regular 28 day periods it is day 14. Some of the period apps are pretty good.