When using the Calendar, single instances of a reoccurring event should be able to be marked complete.
When using the Calendar where I have set an Event to be reoccurring
forever, I would like to be able to mark the current instance of that
event as completed so a strike through line appears through the event
that has been completed but not affecting the later events that haven't
occurred as yet. This would be very helpful as you can get a instant
indication when looking at the Calendar that an event has been done
otherwise you have to go drilling into the event to see if it has been
done. I tend to use this to show reoccurring payments and I don't always
remember if someone has paid or if I have paid a bill etc.