To steal mercilessly from last year's post:
For those who are new to the Stack Exchange network, Winter Bash is an annual light-hearted event where users can earn virtual hats by doing certain things on an SE site within the Winter Bash time frame -- these hats that you earn can be worn as adornments on your SE avatar either on a particular site (e.g., Coffee) or network-wide (i.e., all of Stack Exchange).
The conditions for earning certain hats are "public" -- well-known to all; other hats are "secret" -- you need to figure out what to do to earn the hat. Some public hats might be... ask a well-received question, improve a question, submit an accepted edit, perform tasks in the review queue, etc. Some non-public hats in the past have been... contribute on a particular day, upvote and comment/improve on a first-question from a new user, etc.
Winter bash can be fun and users will sometimes get hats just by participating normally. Any user can opt-out of the Winter Bash (once it starts, any user can click a link like "no hats for me, thanks"); no hats will be earned nor viewed.
Once Winter Bash begins (19 December), stats can be viewed at http://winterbash2016.stackexchange.com/.
As a community, we can opt-out of Winterbash, but we are opted in by default. So, hats: yea or nay?