It started, innocently enough, with a snowball fight. I was casually wandering through Lion's Arch, admiring the hovering Asuran snow-making machines, when I was pelted by something soft and cold. Another followed the first, itself followed by barely concealed giggling. But this wasn't a player picking on my diminutive Engineer. Instead, NPCs and players were making the most of the snow, in a way that made it difficult to tell which was which.
And this is just the surface. Putting aside some snowball-fight based Turing Test; ArenaNet has packed so much into their Wintersday event for Guild Wars 2 that it'll survive even the deepest winter.