Most come from the pagan yule festival actually. Bonfires (now in fireplaces usually), decorating with holly, mistletoe, and evergreen trees, feasts, and gift-giving are all from yule (we’re missing ritual sacrifices from Christmas though, unfortunately). Saturnalia is primarily feasting, drinking, and gift exchanges. There’s definitely some overlap between the two, but yule has a much bigger influence (which isn’t surprising since almost all the other Christian holidays are also pagan based rather than Roman)