Add one
Booking Page → Announcement. Write the message and turn it on. The message is the only thing required.
Optional display window
You can set a start and end date so it appears and disappears on its own. Leave the dates empty and it shows until you turn it off.
Turning it off
Turning the announcement off hides it from your page and keeps the text, so you can reuse it later.
Good uses
Holiday hours, a temporary closure, a new service, or a waitlist note while you are fully booked.