To set up a day as closed, access the configuration of the center (Planning→ Centers configuration).
Then we will click on Center schedules on the left and then click on "New".
We will then select the option "daily", untick the option 'Open' and select the day and month from the drop-down menus.
Once done, click on Update so that on that day the centre will appear as closed and the software will not allow any bookings.