My calendar app no longer shows Done in the upper right hand corner. Instead it keeps Add, even after you click on it to start your new event. As a result, I can't "save" my new event.
What device?
What calendar application?
Have you tried rebooting the device?
On my iPhone 5s, in the default Calendar app, it shows "Add" while I am entering the details for an event on the calendar. Touching "Add" save the event to the calendar.