If you're having a bigger event and don't want to offer any more regular reservations for that day you can block the day. If you do so, no more online reservations will be possible and the staff will also see a conspicuous hint that the day has been blocked and no more reservations shall be accepted.
How to block a day?
Just go to the days page and click the write note or edit opening hours link.
Then click the Edit opening hours link to the right.
Now you may click the checkbox to Lock this day