If i had my period on april 20 and my cycles are 28 days when is my next ovulation days?

14 d before: Ovulation usually occurs 12-16 days before your next cycle, if you have a regular monthly cycle and you ovulate regularly the calendar is not the most accurate method of predicting ovulation, and remember you can get pregnant up to 7 days before expected ovulation.

14 days before: Ovulation occurs consistently about 14 days before your next cycle. If you know how many days there are from the first day of the bleeding to the next during your normal cycle just subtract 14 days. This allows you to predict when you should be ovulating. If you want to avoid pregnancy at your age please do not rely on ovulation predictions.