For multi-day events visitors will have to present a valid Covid certificate every time they access the event location. Spending the night at the campsite is allowed on condition that your certificate is still valid.
If your certificate should expire during your presence (e.g. a test certificate that expires at 10 am) you are allowed to stay until midnight.
If you took a rapid test to get into the event then you will have to do so every day. If the result is negative you can stay on the grounds.
Based on a valid Covid certificate, the organisers of a multi-day event can provide visitors with a means (e.g. a wristband) to indicate the validity period of the Covid certificate. This way you can access the event without getting tested (again) or having the Covid certificate scanned again.