Christmas trees are evergreen trees such as pine or fir that are decorated with colored lights and, often, ornaments. Therefore, it may be appropriate for people who have evergreen trees in their yard to likewise decorate these trees with, say, colored lights. However, it is not appropriate for people to decorate a leaf-less oak or sycamore with colored lights (see dictionary: evergreen).
When an evergreen is decorated with lights, it reminds people of a Christmas tree. The lights are spaced between the needles, giving the tree a pleasant, decorative, ornamental effect. When a non-evergreen (e.g. oak or sycamore) is decorated with lights, it looks like a random string of lights strewn over leaf-less tree branches. Or, sometimes (especially later at night), it looks like a random string of lights strewn in the air. It does not have any recognizable shape and does not remind a person of Christmas. It just looks silly.
I’m no expert at decorating yards for Christmas, but I have seen a lot of Christmas lights during my lifetime. And I’m telling you: do NOT randomly string lights over the leaf-less branches of non-evergreen trees unless you can miraculously give some kind of Christmas-y or truly decorative/ornamental shape to it. Seriously. And even then, please give it some deep consideration.
This concludes this public service announcement. Thank you.