Incubation

They don't really need to be candled at all.
Candling is usually to see if there is development and to see how the air cell is growing.
I don't candle much. I weigh eggs and use that to determine if the humidity is correct. They should lose about 0.65% weight per day if it is right.
Most candle on day 7, 14 and 18.

 

https://www.backyardchickens.com/a/candling-pics-progression-through-incubation-of-chicken-eggs

I was just about to say the days, I see you have gotten info already!

 

It's a personal thing. I think it's best, especially for new hatchers that haven't found what humidity works for them to candle days 7/14/18 to check air cells.... http://letsraisechickens.weebly.com...anuals-understanding-and-controlling-humidity

Some people don't candle or only candle at lockdown, and others candle regularly. (I candle fairly daily.)
And

 

I candle on day 10....Remove any that are not viable....I then candle once a week till lock down...

Cheers!

 

It is true you don't need to candle, plus there is a chance you could drop one, I like to look up websites that show day to day development, and read about day to day development, it helps to pass the time.