Block a whole day or just some time slots (for an event, a holiday or because you are full) from the closures calendar.
Your service periods apply every day. When you close for a day, have a private event or a time slot is already full, close it under Schedule closures and it stops showing in the widget.
You need at least one service period. See Create your restaurant and its schedules.
In My Restaurants, click Configure on your restaurant and then the Schedule closures tab. A monthly calendar opens.

Click the day you want to change. Day schedules opens with every time slot of each period:
Click each time slot to close or open it. Each period has Close all and Open all buttons to close or open all its time slots in one click; to close the whole day, use Close all on every period. Click Save changes.

On the calendar, days are marked Fully closed or With partial closures, so you can see your month at a glance.
Keep in mind that if you change the Interval between bookings in Information & settings, all closures are deleted and you have to set them again.