Hi Nicola,
Wish you luck on your hatch. I'n sure you'll have good luck.
If you put the eggs in the incubator on a Monday morning, then Tuesday morning would have been the end of day 1 and the start of day 2. Wednesday morning would be the end of day 2 and the start of day 3. I'm doing this so we are counting together. If I don't use a calendar, I can get confused.
Stop turning at the end of day 18. I don't think the exact time is that critical as the eggs will hatch several hours apart, but increase your humidity and stop turning at the end of day 18.