I start on December 1st. I do one thing a day for Christmas until Christmas Eve, which is when the tree goes up. I don't do a ton of decorating; a lot of it is baking Christmas cookies or writing Christmas cards. It's just a way of letting Christmas come in slowly, instead of in a huge rush. And it solves the problem of being thoroughly tired of Christmas on Christmas night. So Dec. 1, I put up the Advent calendar. Dec. 2, the Christmas mugs come out. Dec. 3, I bake snowflake cookies, and so on. Everything comes down after 12th Night.
I got the general idea from Madeline L'Engle's book, The Twenty-four Days Before Christmas .