Winter Bash (2014 (promotion page), 2013 (blog post)) is a fun Stack Exchange tradition to celebrate the holidays. Simply put, it's allows users to get "hats" that you can put on your avatar and that are displayed across the network on participating sites.
Speaking of participating sites, Stack Exchange allows each community to opt out as a whole if they have reasons to do so, such as appearing more professional without hats. If you're not a hatter (you're a hater of hats!), you can manually opt out of seeing hats on all Stack Exchange sites.
This event will run from December 14th, 2015 to January 3rd, 2016.
So let's hatve at it! Do we want hats?