
Welcome to the forum and the adventure!

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As they said, incubation starts when they go in the incubator.
Many people count the days wrong. It seems logical that Day 1 would be the day you put them in, but that is not right.
An egg does not have 24 hours worth of development two seconds or two hours after it goes in the incubator. It takes 24 hours for it to have a day's worth of development. Lockdown should be after 18 days of development. Eggs should hatch after 21 days of development. An easy way to tell when they should hatch is that they should hatch the day of the week you put them in. If you started them on a Sunday, they should hatch on a Sunday.
Since you started them on Sunday April 8, lockdown should be Thursday April 26. Hatch should be Sunday April 29.
Just because chicken eggs should hatch after 21 days worth of development does not mean they actually will. Many things like heredity, humidity, how they were stored before incubation, and the size of the eggs can cause them to be early or late. The big thing that can really change hatch day is incubation temperature. If your incubator is a bit warm, they can hatch two or more days early. If your incubator is a bit cool, they can be that much late. It can be nerve-wracking for sure.
If a question saves you from a mistake it is not silly. Feel free to ask anything.