This triggered rule will display a message, if the user is viewing a page of the content type "Exhibition" and the CCK date field is older than the current day:

Add a new triggered rule, call it "Show message when viewing old exhibitions".

Under "Event", select "Node" -> "Content is going to be viewed".

Click "Add a condition" and select "Rules" -> "Numeric comparison"

Copy the "raw date timestamp date" of the field date you want to compare (find it under "CCK date tokens") and insert it into form field "Number 1". Insert the global "raw date timestamp date" in form field "Number 2". Remember to change the drop down between the two form fields to "Less than", like this:

Number 1: [node:field_exhibition_date-timestamp]
Less than
Number 2: [:global:site-date-raw]

Add a second condition: "Viewed content" is "Exhibition", or whatever content type you want this rule triggered.

Add a third condition: "Execute custom PHP code" and enter "return $page;" in "PHP Code:" field, so that the rule is only triggered while you are watching a full node. Otherwise it might also be shown in search results with the content type specified above.

Show a message if the date is older than today: Under "Do", click "Add an action":

"Select an action to add", select "System" -> "Show a configurable message on the site" and enter your "Message", for example: "This is an old exhibition."

Now open up two nodes, one where the date is in the past and one in the future, and verify that the message is only shown on the older node.