Only 100 or so sites mention the legend. here is one.

http://www.christmascarnivals.com/tradition/


According to a legend, the Christmas tree is associated with the birth of Jesus Christ. It is believed that on the night of the birth of Christ, all kinds of living creatures came to Bethlehem with gifts. The olive tree came along with its fruit and the palm with its date but the fir had nothing to gift the newborn king. So an angel, taking pity on the fir, commanded a cluster of stars to shine on its beautiful boughs. Baby Jesus smiled on seeing the lighted tree, and blessed it. Consequently, the fir tree is always lighted with many decorations on it, to please little children during Christmas.