Okay, ready for an alternte view? I have now incubated and hatched with a 100% hatch rate brown egged chickens as well as white egged chicks as well as bantams....
I candle every single day!!!!!!!
Heres why..... 1. it is fun
2. You learn a ton by candeling and seeing the progress every single day, you learn what to see and what to look for. I know some say that if you are new then dont candle until day 9 or 10 but how are you suppose to learn and know what
to look for earlier if you aren't candeling until then? Also, think about how much they are moved around in the mothers hen, she is getting off the eggs at least once a day so opening the bator to check is not an issue at all.
3. I like to get rid of eggs that aren't developing earlier to take the risk out of having one exploding and ruining the whole flock.
4. It is fun
Also, I know this is way way not going to be loved by most, but I wil candle them during "lockdown" as well. I have mine in a spare bathroom so I will turn on the shower as hot as it will go and close the door and come back in 5 minutes. When I take off the top the humidity actually goes up and the temp goes up while I am candeling. If you arent in a bathroom then not a good idea, but I have seen many of my chicks where they have piped through the internal pocket but not outside yet.
So if you are candeling just to see if they are fertile or not then it doesn't really matter when you candle, but if you are like a lot of people and have fun candeling then do it whenever you want and as many times as you want.... IMO