I just upgraded to QGIS 2.18.1 Las Palmas.
This comes with a Santa hat on the icon in the top right corner and windows bar.
Is it possible to remove this from my installation?
Your only real option is to replace the files in:
C:\Program Files\QGIS X version\apps\qgis\images\icons
It's hard coded to change the icon for the 12th month of the year.
I added the feature, and no I didn't consider if someone would get offended by a Santa hat, because:
As it's a simple file replace you are more than welcome to make a plugin to replace the files for people who don't like the icon. I'm also a plugin moderator so if you write it I can fast track the approval for you :)
Thanks to @Snorfalorpagus and @aldo_tapia for the path locations on each platform.