Materials for a group can be hidden from the calendar by saving the material group with "Hide materials in this group on the Calendar" checked.

Hiding a material group on the calendar causes all materials within the group, even those associated with a class, to not be shown in the list of "Materials" when adding an assignment.

This may be useful when materials for one academic term are no longer necessary, 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.