Naruto would probably be really hurt at first. He's always been someone who values his friends and to be forgotten on his birthday would sting. He might try to hide his disappointment though, putting on a brave face.
A plot could also involve Naruto's friends finally realizing they forgot his birthday at the last minute. They then scramble to throw together a party. However, Naruto has already left Konoha feeling dejected. His friends then have to search for him all over the place, and when they find him, they have a heartfelt apology and celebration that not only includes a party but also a promise to never forget again.
One idea could be that all of Peter's friends are so caught up in fighting crime and their own lives that they completely forget his birthday. Peter feels really down about it at first but then realizes that his real purpose is to be Spider - Man and save the city, not focus on a birthday celebration.
Well, he might go through the day feeling really sad and neglected. Maybe he'd see other people celebrating birthdays and it would make him feel even worse. He could start doubting his relationships with his friends and family.
It could be someone like Rock Lee. He was a hard - working prodigy in taijutsu, but often got overshadowed by the more flashy abilities of others. Maybe in this fanfiction, he's completely forgotten until he does something extraordinary to get noticed again.
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 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.
Another reason could be that the author stopped promoting it or abandoned it. If the author didn't keep up with the Naruto fandom's trends or didn't engage with readers, it could have faded into obscurity. For example, if a fanfiction had a very niche plot that didn't appeal to a wide audience at the time, it might have been quickly forgotten as more mainstream - appealing fanfictions emerged.
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.
It could be a new character created by the author. Maybe someone who was born in a small, overlooked part of the village and has a power that is related to the prophecy but has been dormant until now.