Winter is coming...
The Dota 2 map will soon be covered in snow reminiscent of the early DotA All Stars maps that were created on a snow terrain template.
The Frostivus update will be the 2nd time Valve Corporation will be implementing holiday themed cosmetic map changes. (The 1st one was Diretide 2012)
Click on the image to zoom in:
Credits: Matthew Bailey
The map name for the Winter 2012 version is Frostivus.