Yes, you can candle eggs on day 21. This is a good way to check the development of the embryo and make sure that it is healthy and progressing well. Candling involves shining a light through the eggshell to see the contents inside. You should be able to see a clear outline of the embryo and its blood vessels. The embryo should be actively moving around inside the egg. If you don't see any movement, this could indicate that the embryo has died.