You can disable filtering of a category so that its' events are always displayed, even if not selected in a filtered calendar view. For example, schools and universities use this feature for vacation schedules.

To set this option:

Go to Admin Menu >  Categories > Edit or add a category > check "E. Disable Filtering, events in this category cannot be hidden by filters" > Save

Note:  If you turn on this setting in a category, the calendar will always display events in this category even if you filter events by the category or location selection lists.