Day one will be midnight tomorrow & then they'll be starting on day 2. Think about it like this: 24 hours after they were loaded in the incubator they'll have had 1 full day of developing. So, if you set them at 12 AM on May 20, lockdown will be at 12 AM on June 07.
Ive just joined here,and honestly dont have a clue what im doing(not with my hens but the internet on my pesky phone lol)day one begins when you set your eggs,after twenty four hours youre on day two...becky
When you set the eggs, that's day zero for the first 24 hours. Whatever day of the week you SET the eggs is the same day of the week they are scheduled to hatch, the 21st day.