I went to Diamond City the other day and noticed Christmas trees and colored lights all over the marketplace. I thought it was pretty awesome, and looking at my Pip Boy, I also noticed it happened to be 12/25. Neat.
I also saw a post on some forum where somebody visited Diamond City during 10/31 and there were plastic pumpkins everywhere.
This got me thinking, what other holidays (if any) have some sort of special decoration/event/celebration in Diamond City on their corresponding real-life dates?