How do I create a rule to display a system message to users that an event is now over?
Rules can be used for this.  Your message will be displayed using drupal_set_message()

Starting at the Admin screen; 


Click through Configuration > Rules > Add New Rule
Give it a name and select Content is viewed in React on event. Click Save.
Under Conditions click Add condition

Select Data > Data comparison
Select node:type and click Save
Fill in for your content type and save

Under Conditions click Add condition (again)

Select Entities > Entity has field
Data Selector should have node
Field should have the field you're wanting to check. This should be a date field

Under Conditions click Add condition (again)

Select Data > Data comparison
In Data Selector chose your field. Mine was node:field-end-date
Click Continue.
Under Operator select is lower than
Under Data value select site:current-date
Click Save

Under Actions click Add action

Select the System > Show a message on the site entry.
Enter your Message, select the Message type (this is the drupal_set_message() status), untick Repeat message and click Save

Browse to your expired content to see your message.