For a few years now, Stack Exchange has put on a Winter Bash event in which users can earn hats by doing various interesting things and then wear said hats on their avatars. (Kind of like badges, but more visible and lasting only for the duration of Winter Bash.) Other site functionality continues as normal.
This will be AI.SE's first winter. Virtually all Stack Exchange sites will be participating in the hat festivities, and by default we will be too. If you're not a fan of the extra decorations, you will have the opportunity to turn them off by clicking the appropriate button in the footer (which will appear when the event starts in a couple weeks). If there are arguments to be made for disabling hats for everyone on this site, please post them as answers here by December 12.
Otherwise, prepare for hats!