Fireworks are displayed above the ocean, in your mog garden during events; similar to the fireworks that are displayed in cities during the sunbreeze festival. These fireworks also vary per different events (some look like pumpkins during halloween, a star during the starlight festival, etc).

Also if people dont want fireworks to be displayed in their mog garden, they can ask the moogle to turn them off.

I've also thought it'd be neat if during the starlight festival, a "snowing" animation occurred in jeuno to go along with the music.