Each December, the developers run a game / enhancement / treasure hunt that involves hats on avatars.
The event will run from 19 December 2016 up to and including 08 January 2017. The above site will also have a landing page, explaining the rules and other details of the event.
Individual users who don’t want to participate, don’t want to see hats, and/or are generally anti-hat will have an “I hate hats” option available. And just like last time, at the end of the event, all hats will go back into storage.
The only visual change to the Stack Exchange sites themselves will be the presence of the hats and the “I hate hats” button in the footer.
If anyone has a reason why we should hate hats as a site as opposed to individuals, an answer here is a great place to make that case.