Classes for a group can be hidden from the calendar by saving the class group with "Hide this group, including its classes and content, on the Calendar" checked.

Hiding a class group on the calendar causes all classes within the group, including all associated categories and assignments, to not be shown anywhere on the calendar.

This may be useful when classes for one academic term have ended, but you don't necessarily want to delete the group from Helium. Hiding the group on the calendar allows you to keep the group without cluttering the calendar.