The "Premium service" (monthly and annual plans) renews automatically unless you cancel it.
Uninstalling (deleting) the app does not cancel your Premium subscription, and you will continue to be charged. Please make sure to cancel by following these steps:
1. Open the "Google Play Store" app
2. Tap your profile icon at the top right
3. Tap "Payments & subscriptions" > "Subscriptions"
4. Select "Simple Calendar", then tap "Cancel subscription"
On a computer, you can also cancel from the Google Play subscriptions page.
* If you cancel during the annual plan's free trial period, you will not be charged.
* The one-time "Remove ads" purchase has no auto-renewal, so there is no need to cancel it.