What Fools These Mortals Be

A mysterious banner shows up in the Great Hall, sending the castle into a furor of speculation. What is it? Why is it there? What do the tally marks that keep appearing on it mean? What does 'DM' stand for?
Pansy is irritated. Hermione is exasperated. Ron is oblivious. Draco is annoyed. Harry is smug. Will the eighth years solve this new riddle?