Same question as last year. Please do not mark it as a duplicate :P.
I have just got an invitation, asking if our site if we want to participate.
In a nutshell (quoting from the email):
Winter Bash is an annual event that can run on any Stack Exchange site that chooses to participate. Users earn “hats” for their gravatars by completing certain tasks (analogous to badges). Certain actions trigger the user receiving a hat, which their gravatar can “wear”. We track everyone’s progress earning hats in a leaderboard
You can find more if you search for it.