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The Christmas tree is often explained as a Christianization of the ancient Pagan idea that the evergreen tree represents a celebration of the renewal of life. In actuality, when the Roman Empire was converted en masse to Christianity, many cultures did not give up their Pagan ideals and traditions and so they were incorporated into the Christmas tradition.
Many of the symbols associated with the modern holiday of Christmas such as the burning of the Yule log, the eating of ham, the hanging of boughs, holly, mistletoe, etc. are apparently derived from traditional northern European Yule celebrations. When the first missionaries began converting the Germanic peoples to Christianity, they found it convenient to provide a Christian reinterpretation for popular feasts such as Yule and allow the celebrations themselves to go on largely unchanged, versus trying to confront and suppress them. The Scandinavian tradition of slaughtering a pig at Christmas (see Christmas ham), and not in the autumn, is probably the most salient evidence for this. The tradition derives from the sacrifice to the god Freyr at the Yule celebrations. Halloween and Easter are likewise assimilated from northern European Pagan festivals.