Lucky Rocket Cluster

Lucky Rocket Cluster is a type of firework received as a gift from some of the Lunar Festival Elders that you talk to during the Lunar Festival. Using one grants a +250 health buff for 30 minutes.

Source
Each Elder that you talk to during the Lunar Festival will mail you a shortly 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.

Use
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.

Buff
Everyone within a certain range of the launcher when a lucky rocket cluster is launched will gain a +250 HP buff that lasts for 30 minutes (Lunar Fortune).

Object of
Lunar Fireworks 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.