There is nothing wrong with buying clothing, furniture and so on during the seasons of kaafir festivals such as Christmas, on condition that one does not buy things that are used to celebrate the festival or to imitate the kuffaar during their festivals.
In the answer to question no. 69558 it was stated that it is permissible for the Muslim to open his business during the days of kaafir festivals, subject to two conditions:
(i) that he does not sell them things that they use for sinful purposes or to celebrate their festival
(ii) that he does not sell to Muslims things that they may use to imitate the kuffaar during these festivals.
Buying items that one needs is much less serious than selling or opening one's business, and the basic principle is that it is permissible to buy things, and the fact that it happens at the time of their festival does not matter.
And Allah knows best.