The earliest Christian Christmas trees didn't have anything to do with Christ at all, but rather were part of early medieval mystery plays on the Feast of Adam and Eve...which falls on December 24th.
The tree in that story was the Tree of Life, and in these plays, it was decorated with apples to symbolize the forbidden fruit that led to the fall of man.
Over the centuries, the trees began to be associated with Christmas, and by the 16th century in Germany, people were decorating them with candles in addition to fruit.