The History Channel has launched a great microsite about the history of Christmas. Watch videos, see pictures, learn about world Christmas traditions, and much much more.
It was especially interesting to read about the history of Santa Claus and the Christmas tree.
The Wikipedia page on Christmas is pretty informative, but is often vandalized, and there aren't as many pictures. But it's worth a shot.
And don't forget to visit Falalalala.com to refresh your Christmas music collection!