code: def _occursOn(self, myDate):
          
          # TODO analyse which is faster (rrule or db) and test that first
          if myDate not in self.repeat:
              return False
          if CancellationPage.events.child_of(self)                            \
                             .filter(except_date=myDate).exists():
              return False
          return True
docs: Returns true iff an occurence of this event starts on this date
          (given in the event's own timezone).
  
          (Does not include postponements, but does exclude cancellations.)