Here is an excerpt from the General Terms and Conditions for Wacken Open Air 2025 [as of 01.08.2024]:
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).