The end of the first 24 hours is Day 1. If you set them Monday at noon, the end of Day 1 is Tuesday is noon. Day 1 is that 24 hour period.
If I set my eggs today, I count tomorrow as Day 1.
With that said, I have met successful hatchers who turn all the way til the external pip and those who count to the minute and stop turning three days before the ideal hatching date and time.
Personally, even though I have my thermometer calibrated, many of my eggs pip early. Guess my females (chickens, geese and ducks) do NOT have a calendar.
I think that there are guidelines and then people find out what works best for them. I doubt that you'd have a major problem if you stopped turning 24 hours too early or too late. UNLESS the egg hatched and the baby had an accident in the turner.
If you candle on the correct day to stop turning, you should know if you've internally pipped or not. You'll get it down with practice.