Anything before 1st advent. After that it's alright with me.
BUT the christmas tree shouldn't be there until just a few days before christmas. I know a lot of people who get their christmas tree as early as possible, but then they're so tired of it that they get rid of it basically the day after christmas. It's better to get it near christmas, then keep it during new years and a while into January until you get rid of it.
We've got an old saying in Sweden that goes: "Tjugondag Knut städas julen ut!"
It means that you're supposed to clean away all the christmas stuff 20 days after christmas (the Knut part to make it rhyme is just because that is the name that has its nameday that day). Lots of people cleans it away earlier than that, but I try to stick with traditions