When you are fertile.
We have no idea when that will be. Anyone stating which days to try only proves they don't know how it works, and probably thinks the Rhythm Method is a viable birth control option.
You could ovulate today, two weeks from now, a month from now, and so on. We have no idea when it'll happen. That's up to YOU to figure out. Not by app or calendar, because counting days doesn't determine ovulation. You chart your symptoms, sensation, BBT, etc, and figure out your fertile patterns.
In reality, it doesn't matter if every single cycle you've had has been 28 days with ovulation day 14 and fertile period starting on day 9. Anything can still throw off any given future cycle.