1) Go to "Settings > Restrictions > Restrictions for check-in check-out and /or stays > Restrict period".
2) Add name, period of restriction, valid for room type, and all days of the week.
3) Check "Stays not allowed (including check-in)" if you do not want to have any bookings during that period.
The facility can be closed for a certain time period – eg. holidays, renovations, low season or similar. Alternatively, accommodation units can be closed separately, see each unit page.)