This solution involves two files:

• The aspx page that holds the content of the pop-up
• The script that loads the aspx page in a modal dialog upon page load

1. Save this .aspx page to your “Site Pages” directory.
   -OR-
   Copy and paste the following into a new .aspx page in your Site Pages or Pages (Settings wheel –> Site Contents –> Site Pages or Pages) directory:
   
   Wrap this block in style tags:
   [code]
   h1 {
   color: #ed7522;
   text-align: center;
   }
   h2 {
   color: #ed7522;
   text-align: center;
   }
   p {
   color: #1f2844;
   font-size: 1em;
   }
   input {
   font-family: “Segoe UI”;
   font-size: 1em;
   }
   [/code]
   Paste directly beneath (no additional tags):
   [code]
   

   SP Librarian Maintenance

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   We will be performing routine maintenance on SharePoint Librarian                               Monday morning from 4 am – 5 am. There may be a brief disruption for users online during that time. The maintenance window is scheduled for 1 hour.

   [/code]

2. Modify the content in the aspx page beneath the modal-content div and above the input tag to include your own images, formatting and message text.
3. Save this javascript file to your Site Assets (Settings wheel –> Site Contents –> Site Assets) or scripts folder
   -OR-
   Copy and paste the following into a new javascript file in your Site Assets:
   
   Note: Wrap the following in script tags:
   [code] _spBodyOnLoadFunctionNames.push(‘showPopup’);
   function showPopup() {
   var options = {
   title: “Notice“,
   url: “https://sharepointlibrarian.sharepoint.com/SitePages/HomePopUp.aspx” };
   SP.UI.ModalDialog.showModalDialog(options);
   }
   [/code]
4. Update the script to include the URL of your newly saved aspx page and a title for the pop-up window (optional).
5. Add a content editor web part to the page on which you’d like the pop-up.
   1. Edit page
      
   2. Add web part
      
   3. Edit web part
      
   4. Paste URL to the javascript file in your Site Assets and click “OK”
      
   5. Save page/stop editing
      

That’s it! Your pop-up should now function upon page load. When/if you wish to “turn off” the pop-up without deleting the files (so you can reuse later easily) just add “//” before line 7 in the javascript file and save to “comment out” the function. This prevents the pop-up from loading. When you’re ready to use the pop-up again just remove the two slashes and save.

I recommend using SharePoint Designer to easily access and modify the ASPX page and/or javascript file.