Hello Diane,
Thanks for reaching out to us!
You can add the code you need to the single event template by following the instructions within our Themer’s Guide. You can copy the “single-event.php” file into your child theme using the instructions from the guide ( would copy to ‘wp-content/your-theme/tribe-events/single-event.php’) .
Then, under the header there you could use the “has_term” WordPress conditional to check to see if this event is assigned to your desired category. If so, you could then display your desired content.
Let me know if this helps.
Thanks!