To accomplish this, you will want to follow these steps:
Navigate to Administrator → Extensions → Templates Manager and select the template you are currently using as your default.
Navigate to the Offline / Coming soon page tab.
- If you want to use countdown select Show date = Yes.
Add the correct Year, Month and Date
On Content Text field write the info you want.
- Click Save button
Go in System - Global Configuration. In site settings select Site Offline = yes and click save button.
Your site will look like the image below