There’s a date related Easter-egg found in Batman: Arkham Knight.
This Easter-egg can be found rather easily, just change the clock on your PS4 or Xbox One (or PC if you managed to get one before it got pulled) to October 31, and a couple of scenes will play out for you.
However, this Easter-egg can only be unlocked by completing the Creature of the Night mission for Most Wanted and after you have captured Man-Bat. (Hence the spoiler warning)
Video courtesy of Batman Arkham Videos.