When and how often you candle is a personal choice. You don't have to do, but it is a handy tool to check development, weed out dud eggs and detect incubation problems early. I candle 3 times during incubation, first on day 7 to check for fertility and development, before day 7 you won't be able to see much, unless you have very pale eggs and/or a strong light. I then candle on day 10, to double check fertility/development and cull any clear eggs and any eggs that doesn't look right. Lastly I candle on day 18, just before I lockdown to check if everyone is where they should be and if the air cells look like they should. Lockdown is on day 18 for chicken eggs.