Communicating on your site can be a great opportunity to reach new users. You can add a detection script to let people know that you have an app. Here is a tutorial to integrate the detection script.
Once this is done, you should create a section to explain why your festival has an app and the benefits it will give to the attendee.
Here are a couple of examples of website communication:
Bilbao BBK Live 2018
Sonar created a full page to explain important app features.