Here are a few more additions to The Best Places To Learn About Christmas, Hanukkah, & Kwanzaa, this time focused on the tradition of Christmas trees:
A History of Christmas Trees that is accessible to Intermediate English Language Learners.
A slideshow from The New York Times about the lighting of the tree at Rockefeller Center
A slideshow of Christmas Trees at the White House over the years
Another slideshow on various trees and Christmas Tree farms
Better Homes and Gardens shows various ways to decorate a tree in this slideshow.