Somewhere around the seventeenth day the embryo's beak starts to turn toward the air cell at the large end of the egg. This is in preparation for the embryo to pierce the inner membrane into the air sack.
The rationale behind discontinuing the egg turning is so that this beak-to-air sack orientation is not disturbed, which would force the embryo to waste time and energy changing position, and possibly finding it impossible to do given the lack of space within the egg.
Since some chicks hatch early on Day 19 (as in our last hatch) and some on Days 20 and 21 (or later), it is usually recommended to stop turning on Day 18. If you had a way to observe exactly what was going on in there, you could be more precise. But we just do the best we can!
Actually, I have seen studies that indicate turning is unnecessary after day 14-15, but I would stick with the tried and true 18 day mark until more evidence is presented.
