Each Elder that you talk to during the Lunar Festival will mail you ashortly thereafter. Lucky Rocket Cluster is one of two alternate gifts contained in the Lucky Red Envelope. The odds of getting either gift appear to be even.
A Lucky Rocket Cluster must be used near a Cluster Launcher. Conceptually, it is thrown into the Cluster Launcher, but there is no targeting involved, just right click on the Lucky Rocket Cluster in your inventory. See Notes for details.
requires that you launch eight Lunar Fireworks (color and size not specified) and two Cluster Fireworks (color and size not specified). Lucky Rocket Clusters are a Cluster Firework, so firing off one of these satisfies one "Lunar Fireworks Cluster Fired" objective, but note that the cluster fireworks vended near the Lunar Festival Harbinger that give you this quest are cheap, and the Lucky Rocket Cluster buff has value. You may not want to waste any Lucky Rocket Clusters on doing this quest, and especially not two in close succession, where one buff is completely overwritten.
Unlike many fireworks in WoW, Lunar Festival Rocket Clusters (any size, any color) require the presence of a Cluster Launcher object on the ground in order to use the firework. These are widely available during the Lunar Festival only; near Lunar Festival revelers in cities and in Moonglade, and at other odd locations (there's some at the fishing dock in Orgrimmar). You can also spawn your own Cluster Launcher for 30 minutes by using a , made by an engineer from schematics purchased from Valadar Starsong in Moonglade during the Lunar Festival. The schematics cost 5 . The Cluster Launcher item does not bind and you do not have to be an engineer to use it. It has only one charge and is used, not equipped, so it is Consume on Use.
Because the Lucky Rocket Cluster gives its benefit to all nearby when it is launched, the item can be handy for placing a Cluster Launcher object just outside of an instance to buff a raid prior to entering the instance.