Lock down at beginning or end of 18th day???

Don’t worry too much about turning. Turning isn’t really important after 14 days. Turning won’t hurt after that, it’s just not necessary.

But to answer your question, after. An egg does not have a day’s worth of development 2 seconds or 2 hours after it is put in the incubator. It takes 24 hours to have a day’s worth of development. Lockdown is after 18 days of development. Hatch is theoretically after 21 days of development. An easy way to check your counting is the day of the week you start the chicken eggs is the day they should hatch. If you start them on a Monday, they should hatch on a Monday.

The reason I say the 21 days is theoretical is that there are several things that affect exactly when an egg will hatch; heredity, humidity, how and how long the egg is stored, and just basic differences in the individual eggs. A huge factor is average incubating temperature. If it is a bit warm, they can be a couple of days early. If the average incubating temperature is a bit cool, they can be just as late. The 21 days is just a target. If mine hatch within 24 hours either side of the 21 days, I figure they are right on time.