Eager Trophy and Achievement hunters likely already know the deal: today is Halloween and Calendar Man, a Batman villain obsessed with holidays and dates, is looking for a little company. If you talk to him in Batman: Arkham City
on twelve different specific holidays (you can see the exact dates on a calendar next to his cell in the basement of the courthouse), you can earn the Story Teller Achievement/Trophy.
Of course, you could cheat your way through this by changing the date on your console, though we'd hardly condone that. The Batman is not known for taking shortcuts