By simply disabling (deselecting) all under Calendars, you’r left with just the holydays and Nineteen Day Feasts, as these are inserted into the list regardless of any personal calendar-events.
A handy “feature” when you e.g. work on a list of community-activities or the like, and need an updated list of these days.
Select which calendars to use
When all the red checkmarks are unchecked, you are left with a list similar to the one on the left.
You can select which calendar to use in the Bahá’í Calendar app using the “Calendars” menu, but modifying calendar-details like subscriptions, sharing, color etc.. must be done using the build-in Calendar app as of now.
Modifying Calendar accounts is done in Settings on the device under Mail, Contacts and Calendars.
For more help on this subject, and more insight into the possibilities, see Apples’s user manuals.