Christmas tree's and all decorations if follow the true meaning of the festival would be to put this up on Christmas eve, and take down on Jan 6th, as christmas doesn't really begin until the midnight on 24th and the christmas season continues until candlemas Feb 2nd,
It's supposed to be unlucky to keep decorations of any kind up beyond the twelfth day of Christmas - January 6th.
I don't know about twelve days before. Mine go up a few days before Christmas, no earlier than about a week before, and are usually down again by the time I go back to work in the New Year.