When you set your eggs you have 0 days of incubation. 24 hours later you have 24 hours, or 1 day, of incubation. So the next day after first setting them is day 1. It's easy to remember if you think of it like this: The day of the week that you set should also be the day of the week that you hatch. So if you set your eggs on a Saturday, day 21 (hatch day) will also be a Saturday. It's not hugely important what time of day you set them, as day 18 (for lockdown) and day 21 (for hatching) is more a guideline than a dead certain thing. Chicks don't wear watches and will hatch when they are ready, which isn't always bang on schedule.