My app needs to know the user's religion to show the appropriate holidays in their calendar. For example, it won't show Christmas if the user is a Muslim.
The problem is that users may find it very strange that a calendar web app wants to know the religion, and it might even lead to users thinking WTF and they'll leave the app.
Is it good to ask this in the registration form, or is it better to put this in the account settings and let the user discover this themselves when they some time edit their account (and show all holidays by default)?