Why is the song called the 12 days of christmas

it should be the 24 or 36 days of christmas
when we where young
they allways put ours up 12 days before and taken them down on the 6th jan 12 days after and so did we when the kids where at home but now they have left and there is only Anne and me i will leave it to Anne to say when they are put up and taken down
and find somewhere to hide while she does them.......
