We did this last week. It seemed very late in the season for us to be decorating, but our Norwegian friends were shocked that we have decorated the tree so early! Mostly Norwegians decorate the tree on Christmas Eve (Julaften). Years ago, the parents would decorate the tree after the kids had gone to bed and it would be like magic on Christmas morning. This was especially important, because the kids got very few presents, maybe a sweater and a small toy.
Nowadays, people decorate the tree as a family on Julaften, or more and more, on lille Julaften (little Christmas Eve, Dec. 23).