I wouldn't recommend using a calendar or rhythm method as birth control. Not reliable at all. ANY time in your cycle that you have unprotected sex, you are risking pregnancy. Cycles can change at any given month without warning and you can ovulate off pattern. Too risky!
So, there is NO safe time to not use true protection. good luck
First count from your last period to your next period if its 28 day cycle then you are not safe from cycle day 9th to 16th or get an ovulation calendar to know when you are safe.