Everyone has their own tradition, I put mine up on the 7th of Dec.As long as you know the tree is fresh,and you keep it watered you can put it up when ever you want to.Before you do you should make sure that it has been soaking in water with a little bit of sugar in it for at least 2-3 days.Check the water every day,and make sure the needles are moist.
I like to put it up the first week or two in December, I don't like to put it up before then because the tree can dry out and look a bit old by the time Christmas rolls around. I like to make sure it is still a vibrant green on Christmas :)
The best time to put up your real Christmas tree is after Thanksgiving. My family puts up their real Christmas tree on December 1st. The tree is fresh and moist because it was cut in the last week or so. We water the tree each morning so that it stays fresh and moist. If the tree dries out, it begins to die and lose needles. The tree will stay fresh for 4-6 weeks if watered regularly.
Do not wait too long to put up a fresh Christmas tree. The trees tend to dry out while waiting to be sold. A dried out tree could end up dying before Christmas. Check the cut trunk for moisture and freshness before buying. Or go to a Cut Your Own Tree farm if possible, when ready to put up your tree.