The most fun you can have as an Engineer!
Lunar Festival @Wowwiki
This webpage contains a briefing on the various kinds of Fireworks, where you can buy them, and how you can make them. Fireworks are almost useless, but there are lots of fun, if you like that kind of thing.
Fireworks are consumable items, which can be purchaced from a vendor (cheap, most of the time) or from an Engineer who is in the know. Fireworks come in two main types, fireworks, and rockets. The important distinction is that rockets require a launcher to use. Fireworks do not.
Have fun, and try not to catch yourselves on fire!