My period is every 21 of the month. What is the days I can get the highest fertility?

Ovulation timing. The optimal time for conception is 48 hours prior to and up to the day of ovulation. Therefore, start having intercourse a couple of days prior to when you actually ovulate and have intercourse every other day for a total of five times to help ensure sperm are there when you ovulate.
Day 14. Ovulation typically occurs 14 days after your period ends. Good luck.