I find it's always best to be patient and never candle until at least Day 7, better on Day 10 when there will be little mistaking if there is a chick inside. Until you are very adept at it, you will be confused doing it earlier. This is just something that takes experience. Even after hundreds of eggs hatched, I generally try to wait until a week has passed before I candle mine. Also, handling developing eggs before a week endangers them as connections are being formed that are very delicate at that time.