Howeser nailed it. If you set your duck eggs with a 28 day hatch period on a Saturday, they should hatch on a Saturday. If you set chicken eggs with a 21 day hatch period on a Saturday, they should hatch on a Saturday. Many other species hatch periods are not even weeks so that does not work for them.
A way for me to remember it is that an egg does not have 24 hours worth of development the instant it is set in the incubator. It takes 24 hours of development at incubation temperatures before it has developed for 24 hours. That may sound "as plain as day" but it is not obvious to a lot of people. I think part of what confuses it for many people is that the terminology is Day 1, or Day 18. I think the terminology should be the End of Day 1 or the End of Day 18. It seems like the first day the eggs are in the incubator should be the first day but that is not the case.
Howeser made another very important point. The 28 day period for your duck eggs to hatch is a target. Eggs seldom hatch exactly when they should for different reasons. A huge factor is average incubation temperature. If the average temperature is a bit high, the eggs will hatch early, maybe by two or three days. I've had chicken eggs pipping when I went into lockdown with a warm incubator. If the average temperature is a bit cool, they can be two or three days late. Often, especially in still air incubators, the temperature in various parts of the incubator can vary, so it is a good idea to move the eggs around occcasionally when you turn them. Heredity has a bit to do with it. The size of the eggs can affect this. Eggs that are a bit small for the species will hatch a bit early while eggs a bit large can be late.
Hatching is also not an instantaneous process. The chick or duckling positions itself in the egg. It rests. Moving in those tight spaces is hard work. It pips, then often rests a long long time. It zips. Sometimes it rests but often it pushes the egg shell apart before it stops to rest. So you have an exhausted wet pitiful looking baby just laying there. But soon, they are up and running around, drying out and playing rugby with the unhatched eggs.
Good luck on the Christmas hatch.
