When my children were small we put the tree up on Dec 21 and didn't have a lot of other decorations then.
Our youngest son's birthday is the 20th and we were always so worried he would think the tree was part of his celebration if we did it earlier!
Now he is in California and coming home this year and you can bet that tree will be up before the 21st this year.
I now start decorating a bit here and a bit there the 1st two weeks of Dec, then the tree goes up the 2nd weekend with a small tree trimming party.
It is fun hearing of everyones plans and special surprises before the big day