Click 'Site Wide Message' under the 'My Site' heading on the Control Panel menu. Then, click 'Add New Site Wide Message.'
Scroll down to add the details of the message: the dates it will be visible on your website and the message itself.
If the message is ready, change 'Status' to 'Active' and the message will automatically appear within the date range you specified. If you would like to save a draft of the message or remove the message without deleting it permanently, change the 'Status' to 'Inactive.'
When you are finished, click 'Save.'