its best to candle once before incubation (check for cracks, air cell in right place, ect.), once on day 7, and then on day 18.
but i do it daily because i have OCD and i have a fear of leaving no good eggs in the incubator
I did quite a bit, including after day 18 lol, and saw the internal pip. It was very interesting to watch the changes. I only had one early quitter and one that hatched a day early and didn't make it.
I had 2 peacocks left in the incubator last week when the cochins were hatching, so I was opening to turn them and just candled them again at day 20. We heard cheeping in the egg, twas great.
My ocd stems from being convinced I'm going to bake them all, and when my broody hatched a long staggered hatch last month, there was a lot of interuptus going on with those guys, and every single one of the 17 hatched. this included one that had pipped and she got off it for her daily venture. So I'm fairly relaxed on the typical incubation rules
I wash the slightly tinged eggs, put them in, and if I don't have any problem eggs IE weeping or going bad. Then I wait till day 18 to candle right before they go into the hatcher, no need to do it anymore than that IMO. I also set more eggs at the same time one batch goes to the hatcher another goes in the bator.
Folks who do the daily candling is such a waste and can really reduce their ability to hatch, I also never candle after day 18 to risky.
Originally I candle the eggs to make sure there are no cracks in them. I did hatch a chick before from a cracked shell. I used to candle my eggs at day 10, then I started candling at lock down. Now I usually only candle when I load the incubator and after the hatch is over to see why any eggs left in the incubator didn't hatch. Sometimes during incubation I will pull some eggs out and candle them. When I was using my LG styrofoam incubator I did candle more often but now I use a Sportsman and it holds 270 eggs so that is why I only do a few, that way I only open the door when I take the eggs out of the racks to set them for lock down. I wish I had a hatcher, maybe someday. Good luck and have fun!!!