I wrote the plugin that did the happy new year message, and which also triggered the fireworks.
How did it trigger them? It placed a single redstone block. That's all. It's about as simple as a plugin can get, and we only used it to get really precise timing for all the midnights around the world.
The fireworks display (like the entire build) was all survival, and all built by players. The names that you see on the signs around the centre of the new year area are the names of everybody who was involved.
I was blown away by the fireworks. The sheer hard work put in to the show, and the build itself, is like nothing I've seen on any server I've worked on. I'm immensely proud of this community, and I'm really glad I got to be a part of it.