When exactly starts a day of arrival? At midnight?

4.3 When booking the Access Pass to the W:O:A Campground, Camper-Park, Bauer Uwes Garten, Wheels of Steel and Camping Only, a binding arrival day must be selected.
An arrival day begins at 9 AM and ends at 9 AM on the following day (e.g. Monday arrival day begins on Monday at 9 AM and ends at 8:59 AM on Tuesday).