The answer is because in Poland it is believed that a spider wove a blanket for baby Jesus.
Explanation:
Spiders are a symbol of goodness and prosperity during Christmas in Poland. It is common to see spider webs and spiders as Christmas tree decorations in Poland. It is also a very common folktale in the Western Ukraine. Apparently it is also believed this folk tale explains the origin of tinsel on Christmas trees, which is more common to Britain and American households as decorations for the Christmas tree. Finding a spider in a Christmas tree is considered good luck in Eastern Europe and so is finding a real spider's web.
