Rocksteady has a history of giving date specific Easter Eggs in their Batman games. In Arkham City, Calendar Man would change what he was saying based on your computer's date on top of a few other little tidbits. Now a Halloween specific easter egg has been found in Batman: Arkham Knight, which is actually when the game takes place in world.
Some minor spoilers ahead, so I'll throw up the warning sign, just in case!
If you set the date on your console to Halloween, you will see that the ManBat has escaped from the chambers you lock him in at the end of his quest specific storyline. There is some new dialogue too talking about the escape
Take a look:
I'm convinced there are more date specific secrets hiding in this game. We will have to see what the internet finds.