For those of you who don’t use this terminology at your camps:
In-Day: The opening day of a session, where campers are checked in and the first day begins. There’s plan to organize and prepare for in-days.
Out-Day: The day they all leave. At my camp, we (used to) get up at about five in the morning and power through packing, cleaning, eating, and more cleaning. No time to organize or prepare.