I've talked to some people who were under the impression that Christmas Day is the 12th day of Christmas, which would make the 1st day the 13th of December.
Because so many people decorate so early, a lot of people have forgotten the tradition of putting up and decorating your Christmas tree on Christmas Eve. That's because Dec 25th is technically the start of the season, running through Jan 6th.
I certainly have not forgotten. My poor father went out Christmas Eve after we were asleep to buy and decorate the tree. We were told that Santa put up the tree.
I don't think that had anything to do with the 12 days of Christmas though. Although I am 85-90% Celtic descent (Irish, Scottish, Cornish) we had no ethnic traditions, and certainly no religious one despite Epiphany, about keeping the tree up for 12 days. It stayed until the needles fell off.