To some people, day 1 can be the day you put the eggs in. To others it can be the day after they have had a full day of development. Or you can look at it as they are on their second day of development or they have completed two days of development. I can see why people get confused. When people talk about which day, think of it as the end of day whatever.
There was just another thread on here about this that would have helped and I cannot find it. There is a simple way to look at when the eggs should hatch. If you start the eggs on a Wednesday, they will hatch on a Wednesday. (I stole this from that other thread)
Chicken eggs take about 21 days to develop and hatch. Some are a bit faster or slower but about 21 days. The eggs have not had 24 hours of development the instant you put them in the incubator. It takes 24 hours to get 24 hours of development. If you have a photo that says, when you candle at day 10, the development should look like this, that is after 10 full days of development. It is not after 9 days and 1 second of development. If you are looking at hatch day, it is at the end of 21 full days of development.
It can be a bit confusing. Hope this helps.