Here is a ruling from IslamQA:

The Christmas tree is one of the symbols of the Christian festival and celebration; this is why it is named for Christmas. It is said that it was first officially used as a symbol in this manner in the sixteenth century in Germany, in the Cathedral of Strasbourg in 1539 CE.

It is not permissible to imitate the kuffaar in any of their acts of worship, rituals or symbols, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever imitates a people is one of them.” Narrated by Abu Dawood, 4031; classed as saheeh by al-Albaani in Irwa’ al-Ghaleel, 5/109

So it is not permissible to put up this tree in a Muslim house even if you do not celebrate Christmas, because putting up this tree comes under the heading of imitating others that is haraam, or venerating and showing respect to a religious symbol of the kuffaar….