Teloch asked: Birthday fluff.
Zuzu always loved his birthday as a kid since it was the one day of the year when Azula wouldn't tease him at all and would instead just let him enjoy himself, play with him, and she always managed to find the best gifts ever. She'd go right back to teasing him as soon as the day was over, but hey, it's not like he always hates being teased by her, just when she goes too far/really is trying to hurt his feelings.
Azula loves her birthday because Zuzu spends the entire time with her, even sharing her bed the night before, and she's allowed to tease him "without him complaining" (he tries to not complain, but he always caves eventually, and she loves seeing how far she has to push to break him). She also loves seeing how nervous she gets when he hands her the gift he got for her, because he is so afraid she's not gonna like it, even though she always does.
As for how they feel about it when they're adults, they love it even more. They spend some time with their friends and family, but both days essentially become Valentines Day/an extra honeymoon day for them. They are woken up having their sibling going down on them and then they enjoy a romantic breakfast together, get extra clingy and sweet (yes, that is possible) and Zuko gives super sappy and dramatic speeches about how much he loves her and how happy she makes him, and she just melts.
They give each other two gifts: one "normal" gift, and another that they're gonna use in bed at night.