I know that somes stupid.. But does it mean they need a FULL 21 days to develope. Or just 20 days. Please be patient with me I'm having a hard time here..
21 days is 3 weeks. I always figure that if I set eggs on a Friday I can expect them to hatch on the third Friday following. I stop turning and raise humdity on the Tuesday evening prior to that third Friday. Development time may vary if the temp isn't quite right. Too warm will hatch early and too cool will hatch late.
Also, if you set them late in the evening, count that day as day 0 instead of day 1. If you set them in the morning, count the day you set them as day 1.