Lockdown on day 18. It's 3 days before day 21 just in case you have an early hatch like you did. If they hatched early last time, your temperature was probably a tad high throughout incubation. This happened my last hatch as well. It won't hurt to stop turning your eggs a couple of days earlier than day 18 if you're worried. But the humidity doesn't need to be adjusted until day 18, and if by chance you get a pip before that, up your humidity immediately.