aCalendar (free version only) uses ads from Google admob and/or Facebook Audience Network.
You can configure whether you want ads to be personalized or non-personalized or remove ads.
For more details see Google advertising privacy & terms and Google ad settings.
Facebook Audience Network
If your app version uses Facebook audience network, there are no ad settings available in aCalendar. Please go to the Facebook app to configure ad and privacy settings.