A site notice is a useful way to tell customers that you are closed and not accepting orders (Christmas Holidays for example).
Start by going to Site Notices Under Settings:
Choose dates that a site notice will be visible
This is great if you are away on holidays specific dates and will not be delivering and need a site notice ONLY during those periods.
Frequency of the site notice visible
Then check your website to make sure that the site notice is active for you!