Kagiya
鍵屋
Definition
This is the name of a popular fireworks creator. People would shout the name when shooting up fireworks, and it became synonymous with enjoying fireworks.
Example
In Hibiki or any other summer episode where they shoot up fireworks, it's pretty common to see them shout this phrase.
Related Phrase
Tamaya is used for the same purpose.
References
http://www.jmode.com/madeinjapan/madein5-3_2_2.html
http://park.org/Japan/Hitachi/nippon/hanabi/e/ghrj0200.htm