The legend says that fir trees became Christmas trees in Germany due to St. Boniface ("the apostle of the Germans"): n 722 St. Boniface, went to Germany as one of the numerous Irish/Scottish/English missionaries. Whilst there he found a group of pagans performing a religious ceremony around a big oak tree. (In Geismar, now Hofgeismar.) They appeared to be sacrificing a child around it. To stop the ceremony, St. Boniface cut down the oak tree. As if by magic, a fir tree grew from the place the oak had been. St. Boniface regarded this as a triumph of Christianity over Paganism, and the introduction of Christianity into Germany.
Another legend says that Luther was the first to have put lights on the fir tree: he walked through a dark forest one winter night, looked up at the sky to see twinkling stars shining through the branches of the fir trees. He thought the stars to be lights from heaven that were guiding him home, through the dark, frightening forest, similar to the Star of Bethlehem
leading the Three Wise Men to the stable. So Luther took a small fir home to his family, and decorated it with candles to mimic what he had just experienced in the forest. This experience also inspired his next sermon, and saw him spread the idea to his community. It is said that this is why people began putting candles on their Christmas trees ...
The popular christmas song "O Christmas tree ..." is actually about fir trees - thus the original German lyric says O fir tree, O fir tree, how faithful are your branches! You’re not just green during summertime, but also in wintertime when it snows, o fir tree, o fir tree, how faithful are your branches!