I candle on day 22 or 23 only. The way I see it, if only one of the eggs out of ever how many I set, is living, the incubator is going to be on for the entire 21 days. I have had eggs i thought where not developeing when I candled early, only to hatch on day 21 anyways. So much for my candleing skills. If the incubator is going to run for 21 days anyways, why throw out any possible's or maybies. When the eggs hatch, they usually hatch all at the same time or within just a few hours of each other. If after a majority of the eggs have hatched and I still have a few that havent, I will candle to see whats going on, and only then start throwing eggs out. I have gotten live chicks out of the trash after thinking the egg wasnt going to hatch. My wife reminds me of that one everytime I start getting antcey about emptying the bator. Nothing like hearing, "are you sure? are you sure? are you sure?" , over and over again.