I am sorry to spoil your preparations for Christmas before the Christmas lights have even gone up—though perhaps it is better to do this now than the week before Christmas, when everything has been carefully prepared. But Jesus wasn’t born in a stable, and, curiously, the New Testament hardly even hints that this might have been the case.
So where has the idea come from? I would track the source to three things: issues of grammar and meaning; ignorance of first-century Palestinian culture; and traditional elaboration.
The elaboration has come about from reading the story through a ‘messianic’ understanding of Is 1.3:
The ox knows its master, the donkey its owner’s manger, but Israel does not know, my people do not understand.
The mention of a ‘manger’ in Luke’s nativity story, suggesting animals, led mediaeval illustrators to depict the ox and the ass recognising the baby Jesus, so the natural setting was a stable—after all, isn’t that where animals are kept? (Answer: not necessarily!)