Winter Bash is coming up again next month. See 2013's site for more info, in case you haven't heard of this: http://winterbash2013.stackexchange.com/
Different SE sites can choose to opt in or out. It's fairly harmless; just adds some cosmetic effects (hats) to the site and 'achievements' to earn hats. Before opting in, does anyone have an opinion on whether to enable it for datascience.stackexchange.com or not?