Normally, your tax advisor should tell you how you are reporting revenue and how you are taxed.

If your revenue is taxed at the time of issuing the invoice, you should base yourself on closed invoices.
If your revenue is taxed at the time of consumption by the guest, you should base yourself on consumed revenue.

Most hotels I know do based on consumption, while tax rules can be a minefield per country and situation, so you should check with your tax advisor what the applicable rules are.

Thanks a lot Marc for your quick and helpfull reply!