This doesn't seem to be talked about in the anime or even in the manga. We just see the 3 other student council peeps meeting up in the background of Kaguya's thoughts:
This however is brought up in the movie:
Chika (Fujiwara) asks this.
Miyuki responds claiming to always wanting to have awareness of being president of their school's student council.
Yu (Ishigami) asks (jokingly? semi-jokingly?) if it was because Miyuki didn't have any fireworks attire or something.
Why does Miyuki REALLY attend the fireworks festival in school uniform? Seems like we can only speculate, but I'm wondering if perhaps some native Japanese or at least people familiar with Japanese culture will be able to shed some light here. Does Miyuki actually have no, what do you call it, yukata or something? Or what?
It is indeed weird or something to attend the fireworks festival in a school uniform, right? Or what?
Edit: Oh yeah this definitely should've been split into 2 questions. Anyway, as usual, I'm just punishing myself by bounty-ing 1 and accepting the other.