I have been to a few spontaneous dance parties around the game, haven't been to any big public events. There was the anniversary event stuff but that was more fighting mobs and hunting for loot than spontaneous dancing. This was pretty cool though, last night while greeting with Velcome a wild dance party broke out! Here are some pics:
Best of all, most people dancing were not in a guild and new to the game.
I think we will be having more of these.