It is the decor of your room. In the location where you want to and think are appropriate or may be used in the party. Christmas day, or by the various festivals that you provide it.
It will be decked out in your room. In the position you want and think is appropriate. Or you could take to the party or the Christmas festivities that you held it up.
It is decorated in your room. In a position where you want and think proper. Or you may be used in the party, Christmas or the festivals you set it up.