So far I've set off all the known fireworks, seen the various firework encounters, beaten the Intertwined Serpents and Firework Spider, and also done all the trading in Fireworks Trade and at least returned the plush for Tiny Choking Sobs, so I should be past the requirements for the Grand Finale based on previous years, since the Smoke Dragon hasn't seemed to be required (and indeed can be fought after the finale). I did set off the smoky firework, though.
Even with all of that, though, I'm still seeing regular fireworks displays, not the finale, so it seems like there's something else going on to find. The bookstore and Zaibatsu don't seem to have anything, and I've grabbed stuff from the mailroom a few times just in case, but no unusual finds there. I also tried crafting dry ice and glass of red with a few fireworks just in case, being that they're new-ish items.
Edit: Looks like it was an issue, rather than a missing step. Finale is now available.
