I believe after the eggs have been in the incubator for 20, 24 hour days. So if you put them in at 8 pm on a Sunday, the next Sunday at 8 pm will be 7 full days, the next Sunday at 8 pm would be 14 full days, and the next Saturday at 8 pm would be 20 full days, 8:01 pm on that Saturday would start day 21 and day 21 would end at 8 pm on Sunday.
I have found this question asked alot all over this forum. It seems the answer is varied depending on who you ask.
Some say the first 8 hours don't count, some count it. Some say day one is 24 hours after the incubation starts, some day it is the day it starts.
Personally, I am did a broody hatch and am currently in the process of a incubator hatch. I will figure out the results from those and determine my 21 day count from that.