Here is a quote from Wikipedia's page about the history of this ceremony...

"The practice of "topping out" a new building can be traced to the ancient Scandinavian religious practice of placing a tree on the top of a new building to appease the tree-dwelling spirits of their ancestors that had been displaced." -- http://en.wikipedia.org/wiki/Topping_out

So, (if we're going to take history at its word) it doesn't even have anything to do with Christmas in the first place.

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