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.