How to force show an article even if it is unpublished?
If an article in Joomla! has been marked as unpublished or the "Finish Publishing" date has been passed, the Joomla mark the article's link as a 404 error. So, we have to display the content of the article in the modal dialog's way.
1. Navigate to the Components > Monthly Archive > Settings
2. From the Main Settings, scroll down to the section "State" and select the option "Show" for the field "Show Unpublished".
3. On the same page, click on the tab "Layout", scroll down to the section "List of all articles of a Year or a Month" section, and select the option "Modal dialog" for the field "Link Type".
4. Save options, and now you are able to show the unpublished or the expired Joomla! articles inside the Monthly Archive menu items.