You want to get it done at 7dpo. When they say day 21 they assume O happens at CD14 (ie textbook 28 day cycle).

Are you tracking your cycles (ie opks/etc and/or temps)? If so, you'll have a good idea when you O'd and you can go 7 days after that.

If you're not tracking your cycle, I'd go 7 days before AF is due. It may be not completely accurate (eg if you have a short LP), but it will be much closer than CD21.