
Despite the popular song, the 12 days of Christmas don't lead up to December 25th — they start on it. The twelve days run from Christmas Day through January 5th, the eve of Epiphany, historically a period of continued celebration rather than a countdown.
In many traditions, this is when decorations stayed up and gift-giving sometimes continued in smaller ways each day, rather than everything happening in one burst on the 25th.
It's a nice excuse to leave the tree up a little longer — you're not behind on taking it down, you're technically still celebrating.