Rocksteady's no stranger to putting in Easter eggs based on dates in their games. The Calendar Man features in Arkham City an example of this. Batman: Arkham Knight has one involving Man-Bat.
Spoilers so if you don't want to know, don't watch the video. The Man-Bat was one of the cooler things about Arkham Knight and I enjoyed Rocksteady's interpretation of him.
The Easter egg involves setting your PC's time to October 31 and grappling around to the top of buildings. You'll see a quick sequence involving Man-Bat and then you'll need to head back to GCPD to see what happened.
I hope there are more of these in the game.
