Maybe in that fanfiction, Naruto grew up in an isolated or deprived environment where such concepts weren't introduced to him.
Perhaps in this particular fanfiction, Naruto was so focused on training and survival that he never had the chance to learn about something as mundane as birthdays.
It could be that the author of the fanfiction wanted to create a unique backstory for Naruto where he was deprived of normal childhood experiences, including knowing about birthdays.
One idea could be a time - travel fanfiction where Naruto's friends go back in time to his birth and make sure his early years are filled with more love and support. They could prevent some of the early hardships he faced.
In some Naruto birthday fanfiction, there are often big parties. All his friends from Konoha, like Sasuke, Sakura, and Kakashi, gather to celebrate. They give him unique gifts. For example, Sasuke might give him a new jutsu scroll or Sakura could give him a handmade charm for good luck.
One idea could be that all the characters in the Naruto universe come together to throw a surprise party for Naruto. They could gather in the Hidden Leaf Village square, with Iruka Sensei bringing a huge cake. Sasuke could return for the occasion, and they all share stories and memories of Naruto, highlighting his growth and importance in their lives.
To plan a Naruto birthday party in a fanfiction, start with the setting. It could be during a time of peace in the Naruto universe. As for the party favors, small replicas of the Sharingan or Byakugan could be really cool. You can also include a mini-story within the fanfiction. For example, Naruto could receive a mysterious gift that leads to an adventure during the party. And for music, play some of the Naruto anime's iconic soundtracks in the background to create an immersive atmosphere.
One possible scenario in this fanfiction could be that Naruto's friends all got caught up in some mission on his birthday. He's waiting for them to return, but as the day goes on, he starts to feel that familiar loneliness. He then decides to go to the training ground alone and train, using the physical exertion to mask his disappointment.
One idea for a Naruto birthday fanfiction could be to focus on a surprise party his friends throw for him. They could all gather at Ichiraku Ramen, his favorite place, and decorate it with symbols of Konoha. Each friend could bring a special gift related to Naruto's journey, like a new kunai set modeled after the ones Jiraiya gave him.
To make it stand out, focus on the emotions. Show Naruto's genuine surprise and joy at the celebration. You can also include some rare characters in the celebration, like the old Hokages coming back as spirits to wish him well.
He might meet an old sage who has been living in seclusion near the village. The sage could see Naruto's loneliness on his birthday and offer him some ancient ninja knowledge.
It could be Sasuke. Maybe Sasuke is in a bad mood on Naruto's birthday and rejects Naruto's invitation to hang out or something. Since they have a complex relationship, Sasuke's rejection would really hit Naruto hard.
Naruto might go to the Hokage Rock and sit there alone, looking at the village below. He thinks about how far he has come and yet, on this special day, he doesn't have anyone to share it with. His mind drifts to his parents and how he wishes they could be there for his birthday. He then decides to do something good for the village in their memory, like cleaning up some of the less - visited areas.
This is a really dark concept. In fanfiction, such a scenario might be written to explore extreme emotional distress. Maybe the author wants to show how much pressure Naruto has endured secretly, and on his birthday, which is supposed to be a happy day, it all becomes too much to bear. However, in the original Naruto story, Naruto is a character full of hope and perseverance, so this is a very different take.