I'd have to look, I think it was day 56. I waited for pippers then cut. I had 2 that night so cut the rest. It was one of the pippers came out that night.
If nothing ever changed, there would be no butterflies.
I cut either after the first egg pips or on day 58 whichever comes first. I also leave the lid on and have no air holes. They are usually out of the egg within a day or two after they pip\cut.