Sitewide messages are great for getting your message across if you are going on holiday or have a quick message such as courier delays etc.
There is also an option of adding a header to your page if your template supports it: Add a header
Options are a plain simple message (this has a plain red border), a Formatted box (which you can customise with the content box editor or the dismissable panel.
Go to the Design tab > Sitewide messages
Choose your style of message, plain, formatted or dismissable panel from the message styles.
Choose where you would like it to show from the target location box.
Enter your text and when you would like it to show (if applicable) and Add Message.
You can then choose to edit this message further by clicking on the edit button.