This is an official bonus map for Age of Empires III.
Winter Wonderland Custom Map: a festive new custom map for the holidays! This snow covered map is dotted with red nosed deer, decorated trees, and piles of gifts!
Download the zip file and then simply extract the contents (Christmas.xml and Christmas.xs) to your: My Documents\My Games\Age of Empires 3\RM folder.
Full example path: C:\Documents and Settings\Gsomers\My Documents\My Games\Age of Empires 3\RM