Special Periods can prevent all bookings for a date range, or just visually draw attention to a special period.
Find this screen via admin menu → bookings → lodge availability.
If you tick the box to allow bookings, a special period shows up on the Booking Matrix and elsewhere with a cross-hatched background.
Both types of special periods show the Reason field when the mouse is hovered over the matrix. The Access Level field controls who can see this text – it does not control which access level can book!
If you do not tick the Bookings Allowed box, the matrix shows a solid colour and no bookings can be made except by admins with UNAVAILABLE_PERIOD role.