Holly, apparently, was part of the Roman celebration of the Saturnalia (upon which the Christmas holiday was directly modeled), as it was considered sacred to Saturn. In Celtic countries it played a central and symbolic role in celebrations of summer and winter soltices.
Holly and other evergreens were subsequently adopted by Christians as Christmas decorations in Roman times, despite such customs being disapproved of by church authorities who regarded the decorations as "too pagan."
Now, sprigs of holly seem to be an iconic emblem for this season.