One of the special items engineers can create is various types of fireworks. These serve no actual purpose beside conjuring up some pretty spell effects. If you enjoy that sort of thing, you could put them to good use at ingame social gatherings.
Regular Fireworks
Simple fireworks that can be used without a launcher:
●Green Firework
●Red Firework
●Blue Firework
●Snake Burst Firework
The colors fireworks require 150 skill to craft. The schematics themselves are tradable. They can be bought in the following locations:
Green
Crazk Sparks (Stranglethorn Vale)
Gagsprocket (The Barrens)
Red
Sovik (Orgrimmar)
Blue
Darian Singh (Stormwind)
Gearcutter Cogspinner (Ironforge)
As you can see, the blue firework can only be bought from Alliance vendors and the red one from Horde. You can still obtain all three colors by making use of the neutral auction house.
The Snake Burst Firework can be acquired by both factions by completing the quest: Zapping the Giants given by Zorbin Fandazzle by the docks in Feralas. It requires 250 skill to make.
Firework Launchers
Bigger firework require a Firework Launcher to use. Engineers can craft these, as well as several fireworks to be used with them. Once placed on the ground, anyone can use a firework launcher.
The fireworks we can make for a launcher:
●Small and Large Green Rocket
●Small and Large Red Rocket
●Small and Large Blue Rocket
The small rockets require a skill of 125, the large ones take 175. The launcher itself needs 225 skill.

The schematic for the launcher and matching fireworks can only be obtained during the Lunar Festival. Read more on that further down.
Cluster Launchers
The most spectacular of fireworks require a Cluster Launcher. Naturally, we can craft these as well, again with corresponding fireworks.
●Green Rocket Cluster
●Red Rocket Cluster
●Blue Rocket Cluster
●Large Green Rocket Cluster
●Large Red Rocket Cluster
●Large Blue Rocket Cluster
The regular clusters require 225 skill, the large ones 275. All of the schematics, including the cluster launcher can only be obtained during the Lunar Festival.
Lunar Festival
The Lunar Festival is an ingame holiday event, it happens early in the year and is the only chance for you to quest for most of the firework related schematics shown above.
Look for Fariel Starsong in the eastern area of Nighthaven during the Lunar Festival. Se has a total of six quests for you, one for each type of firework and two for the launchers. You need five Coins of Ancestry to complete each quests, so you'll be needing 30 of those in total.
During the time of the festival you can obtain a maximum of 50 of these coins. It will cost you a fair bit of time as you have to travel all over the place to get them. Note that the coins are found in zones of all level ranges.
As for the how and where to get these coins, I point you to wowiki for the Horde and Alliance versions. Good luck!