I've been wondering about the upside down Christmas trees and decided to search and see what I could find out. I found several places offering background on the upside down tree. One was, ChristmasCarnivals.com which also has many other links for Christmas history to check out too.
“Christmas is associated with many traditions, of which the Christmas Tree is an inherent part. The history of the upside down Christmas Tree has its roots in the 7th century. It is during this period that St Bonafice journeyed from Devonshire, England to Germany to preach the message of God. He engaged himself in religious as well as social work and spent a lot of his time in Thuringia, a town located in Germany itself, which is the birthplace of the industry dealing with Christmas Decorations.
It is believed that St Boniface, while staying in Thuringia, took the help of the triangular fir tree to represent the Holy Trinity made up of God the Father, God the Son and God the Holy Spirit. As a result, the converted people started to [consider] the Fir tree as God’s Tree. Then St Bonafice, using this triangular shaped tree tried to introduce to the pagan tribes the paragons of Trinity.
By the 12th century, it became a custom, especially in Europe to hang the Fir trees upside down from the ceilings to symbolize the Holy Trinity. The Upside down Christmas Trees was also considered the symbol of Christianity. However, the real history behind the hanging of the upside down Trees remains vague. Presently the trend of hanging a Christmas Tree has changed, because nowadays the tip of the Christmas Tree is made to point towards Heaven, as many think that an upside down Christmas tree is a sign of contempt.”