App Store can take up to 2-3 weeks to approve submitted apps. In light of this it is highly recommended to submit your app at least 10 business days before the start of your event, so the app is published on time. In your dashboard, you'll find an indicator of the estimated app publishing period from the current date.
Please keep in mind that it is only an estimate, and your app may be published earlier.
Android app version is usually published within 1-3 business days of submission, 5-7 business days is a more usual turnaround for the Apple App Store. The web app is available as soon as you submit your app.
Publishing your event app early provides you with enough time to promote your app which helps to maximize adoption rate.