That would be a Christmas tree. It's part of an overly complex ritual associated with the birth of the Messiah of a religious group on Earth who call themselves Christians.
Decorating a tree to mark the occasion was started in the 16th century in central Europe, probably in Germany. There's some vague historical records about a thousand years earlier, that a Christian saint chopped down a mighty tree that represented Thor to the local people, in order to disprove the existence of the Norse Gods and convert them. I wonder how the Asgard would feel about that...