We'll do this again, as we've done in previous years.
This year's event will start on 14 December 2015 and run up to and including 03 January 2016. Users will be able to see all the hats they've earned on http://winterbash2015.stackexchange.com. That site will also have an FAQ to explain how things work.
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.