When to candle zebra finch eggs?

Never. There is no need to disturb her to candle eggs, just take good care of the parents and let her care for them. Most people can not ell what is going on in the egg if candling. The best time for me is to candle at ten days of incubation, it doesn't matter when the egg was laid. Candling is for is hatcheries that need every space for good eggs and they remove bad ones to save money.