Fallout 4: Diamond City residents celebrates certain holidays
Fallout 4 decks the galls when Christmas rolls around - in-game.
In Fallout 4, you can check the exact in-game date and time in the Status tab of your Pip-Boy.
So what? So: special events are triggered by the in-game date. Diamond City seems to be the place where the action happens; Redditors have reported guards commenting on Halloween, and decorations go up for Christmas.
Plenty of games have Easter Eggs tied to the real-world date\, but not many foreground the passage of game time so clearly. It won't be long before someone uses the game console to force various other dates and see if there are any other secrets. St Patrick's Day, maybe...?
Note that this isn't why you see Halloween decorations all over the place; that's down to the timing of the nuclear disaster presaging the series.