In theory it is not the beginning of Day 18, it’s the end after 18 days of development. But in reality it doesn’t matter that much. The purpose of raising the humidity for lockdown is to help reduce the chance of the egg drying out when it pips, causing the membrane around the chick to dry and shrink, trapping the chick so it can’t hatch. In spite of what you read on here, that really doesn’t happen that often, but it can. That’s why I consider it good practice to raise the humidity when you go into lockdown, to improve your chances of that not happening. The important thing is to get the humidity up before they external pip.
Sometimes the eggs pip and hatch quite a bit early, as much as two full days. By raising the humidity with three days to go, you get ahead of that curve.
The other side of it is that the egg has to lose enough moisture so the air cell is big enough for the chick to breathe after internal pip but before external pip. That’s usually a pretty big target, how much moisture loss is enough. It usually does not affect your hatch rate if you are several hours off, even a full day. You just don’t have to be that precise.
The reasons you turn them are to keep the yolk or developing chick from touching the inside of the shell where it can get stuck, plus turning helps the body parts to form in the right places. But by about 14 days all body parts have formed and a membrane (the one that can shrink-wrap the chick) has developed to protect the chick from touching the inside of the shell. You don’t have to turn the chicken eggs after about 14 days but we customarily do just for the convenience of doing everything associate with lockdown at the same time. It doesn’t hurt to turn the eggs later, it won’t hurt them, but you don’t have to.
You often read a lot of stuff on here and think you have to do something a specific way or you are guaranteed disaster. This doesn’t work that way. Nature put a lot of leeway in these things to make it easier for the hens to hatch the eggs. Of course you want to be as close as you reasonably can, but don’t overstress about it. I used to be in the army and I once pitched horseshoes competitively. I don’t agree that close is necessarily good enough with either grenades or horseshoes, but with these things and hatching eggs, close usually is good enough.
