I can't seem to manually control the state of "done"-ness for my epics, and it is often wrong.
It seems it would only be "done" when all stories are complete. But this not the case. Perhaps its a bug? Perhaps this happens when all stories are complete, but then I add a story later?
The most frustrating aspect of this is that "done" epics are hidden by default and epics I'm working on keep disappearing.

An epic in Pivotal tracker is done when all its stories are either done or in icebox.
Found this from here