1) Scroll down the Control Panel and click 'Site Wide Message' under the 'My Site' heading.
2) Select your 'Existing Site Wide Message' from the drop-down menu.
3) Click the 'Edit' button.
4) Scroll down, enter the dates you'd like your 'Site Wide Message' to run. (yyyy-mm-dd)
5) Edit your 'Site Wide Message Banner'.
6) If you'd like to launch your site wide message, select 'Active' from the drop-down menu.
7) When you are finished, click the 'Save' button to finalize.