To configure the program to show the center closed on a specific date, access the configuration of the center (planning→ center configuration).

Once inside the center's file, go to Center schedules on the left menu, and press "New".

Then select the option "daily", the untick the option Open (so the program will know that day is closed) and select the day and month from the drop-down menus.

Once you're done click on Update,  and that day, the centre will appear closed and won't allow any bookings.