Winterbash is a competition within SE that allows everyone to win hats by doing various things, similar to badges, but only from December 19th through January 8th, 2017. Any hats can then be displayed on your avatar during this time like this:
The full summary from last year is here.
Some hats will have public descriptions to aim for but some have secret criteria (though there normally ends up being a long thread on meta.stackexchange with a list if you really want to find them that way).
Winterbash is opt-out, so unless there are significant objections, Workplace will be opted in.
Winterbash, yay or nay?