After 10-13 days…in case of 21 be 3 or be 4 is your ovulation day…use my calendar app it will be very useful to identify the days

14 days before menstruation is ovulation period ...you can check simple you tube video for details

i think this chart would help you.

Safe sayhon safer time compared to other enji u could even get pregnant if intercourse is made during menstruation

If you have regular menses you can easily estimate your time of ovulation by substracting 14 from the day of menses...for a example the ovulation time of a girl whose menses is every 28 day is at 14th day.....in your case the ovulation time is at the 7th day