Winter Bash is a special celebration that typically runs from December 15th until January 4th each winter (2012, 2013, 2014, and almost certainly beyond).

Users who participate earn hats for their avatar, via various acts on Stack Exchange sites.

Here's a sample of user avatars wearing hats:

Reference:

• Winter Bash 2012 promotion page
• Winter Bash 2013 promotion page
• Winter Bash 2014 promotion page
• Winter Bash on Meta Stack Exchange:
  • 2012
  • 2013
  • 2014
  • general