Hello,

Well, no such feature for such a part of the event, but if i have well understood, it is some information you add only in the event description ?

If yes, why not using a custom module (joomla module with editor), and then to load this module in a "fack" position (one you will create just for this, but that does not exist in your site template).
This way, you can set the start and end date of publishing for the module with your special offer inside.

And to display it in the event description, just use the joomla feature {loadposition custompositionname}

Is it maybe one solution for you ?

Best regards,
Cyril