On the World of warcraft events page (http://www.worldofwarcraft.com/info/events/newyears/) it says:
"The innovative and artistic engineers of Undermine have been slaving away since their display of pyrotechnics in July. Their latest and greatest -- not to mention highly dangerous to operate -- creations will be on display in the cities of the world every hour after sunset. Keep your eyes to the skies, and be sure to stick around for the finale!"