Hank "Rusty" Truman asked: what Halloween traditions do they have?
As kids, they'd always go around the town trying to get candy together - just the two of them, even though they claimed not to enjoy each other's company (especially Zuko). They walk around holding hands (just because Zuzu is a responsible brother... they're not totally using that as a way to act "like a couple" because all they know about how couples behave is that they hold hands and kiss), and they wear matching costumes because of course they do. Azula takes trick-or-treat WAY too seriously, and her pranks scare her friends and family to death - to the point that she'd often end up being grounded the next day.
As teenagers, since they're both socially awkward/weird kids, they just stay at home, watching movies and eating candy... but still wearing costumes because of course they are, but they no longer need to be matching.
As adults, they throw THE most awesome halloween parties every year, and even Mai can't pretend she doesn't like it. They have the best couple's costumes imaginable.
One thing that stayed the same from the their childhood until the end of their lives was the house having a super elaborate halloween decoration the entire month, and people get legit scared.