So if I'm reading things correctly, the last page seems to imply that the fireworks festival wasn't actually a regular thing. The guy who wanted to do the fireworks festival suddenly got confused why he wanted to do one so badly when they've never done it before.
And the reason is because there was some evil ghost/demon thing that got sealed away by these girls, but the fireworks were a way to unseal the demon (which is why the ghost girls were trying to stop the fireworks from happening), and so when the fireworks happened, everyone who saw them died due to the demon being unleashed.
I guess that's what happened?
Also the detective really isn't normal after all, huh. I wonder when we'll explore what her deal is.