DISCLAIMER: Potterworld or Droobledore LLC is not in any way affiliated with Mojang AB, J.K. Rowling, Warner Brothers or any company, copyright or trademark.
Personally, my family always dresses up together, (like last year we did Alice in Wonderland), so we have to start early, getting costumes as early as late July or early August! It's a lot of thrift shopping so it takes a while, but it's always fun.