I realize this is an old thread, but I would start with a set of holidays that would be considered the 'default' state, and then ask the user if they want to personalize their calendar adding those that are significant for their culture. Sort of what Twitter and other similar platforms do (although I think Twitter forces you to choose people and groups to follow), you can either skip the customization or choose your own holidays.

As mentioned above, any set you try to create will have variations depending not only on religion, but on region as well. But you can't expect the user to pick every single holiday themselves either, so I'd try to make the app as localized as possible, maybe combining region and religion to give an appropriate set of possibilities.

I realize this is an old thread, but I would start with a set of holidays that would be considered the 'default' state, and then ask the user if they want to personalize their calendar adding those that are significant for their culture. Sort of what Twitter and other similar platforms do (although I think Twitter forces you to choose people and groups to follow), you can choose skip the customization or choose your own holidays.

As mentioned above, any set you try to create will have variations depending not only on religion, but on region as well. But you can't expect the user to pick every single holiday themselves either, so I'd try to make the app as localized as possible, maybe combining region and religion to give an appropriate set of possibilities.