Go into lockdown on the 18th day at the same time you set them, so if you set them at 6:00 am on the first, then go into lockdown at 6:00 am on the 19th, but it shouldn't matter too much. I personally would stick with the day 18 even though they started hatching early last time, going into lockdown gives them a 3 day window on both sides to pip with the high humidity. But do whatever you feel is right.