Putting up and decorating is a big family tradition at our house and the annual photographing of the tree is my own sub-tradition.
The photo setup starts long before the camera comes out, beginning with the position of the tree. In addition to the location selected in the house (ours has a very logical one), make sure that the tree appears vertically straight (regardless of the trunk's curvature) and, if the tree is being centered on a feature (such as a set of windows), fine tune that position.
The next item on the checklist must be attended to before you string the lights on the Christmas tree (I know, that is the favorite job). The key is to make sure that all of the strings of lights have bulbs with the same brightness (or that they are dissimilar enough to look proper). Perhaps I'm not the brightest bulb in the pack as I skipped this step (thought we corrected this issue last year) and ... apparently there are two different Christmas light models in our tree kit. At least I have a dark-light-dark-light pattern going up the tree. Hopefully most will think that design was intentional.
After the tree is fully decorated, clean up the area around it – minimally all that will be included in the frame. This task may include smoothing the carpet if it shows tracks. The Best Time of Day to Photograph the Christmas Tree
A Christmas tree can be photographed at any time of day or night, but the best time of the day is when the ambient light is right for the image you want. A tree located in a room with many windows will, without significant shading, show mostly green with ornaments and a subdued look to the strings of lights. This is a very nice look.
My preference for photographing our tree, installed in our great room/living room, is to use only the light from the Christmas tree lights with very low or no additional ambient light. With windows behind our tree, I am quickly limited to the after dark timeframe. After dark comes early in late fall and early winter, but there is another issue. People walking around cause the floor to flex slightly and that makes the ornaments swing, becoming blurred in a long exposure. So, after the kids are in bed (or plan to be somewhere else in the house for a period of time) works best for long exposures sans kids in the picture.
Having windows in your photo quickly complicates the tree photo session for a couple of reasons in addition to the ambient light they provide. One reason is what is outside of the window. Waiting until after dark usually takes care of this problem unless there are other lights visible through the windows (such as street lights). In the latter case, blocking your windows from outside, such as with black foam core, may be a solution to the issue. In this photo, I waited until late in the blue hour on a very foggy day to get a slight even blue glow through the windows. I wasn't sure how bright I wanted the blue to be, but capturing a frame every few minutes allowed me to choose what I thought was ideal at a later time.
Reflections are another issue with windows. If Christmas lights are being reflected, those reflections are often beneficial to the composition. But, if something else in your house is being reflected (such as the LCD panel on the microwave or thermostat), the effect will not likely be appreciated. Lens Selection for Christmas Tree Photography
The desired perspective should always drive focal length selection and in this case, working space may limit the perspective options. Within the bounds of walls and other furniture (consider moving it), the optimal perspective will often result in a wide angle lens
Experiment with perspective, utilizing the various focal lengths at a variety of distances. Also try a variety of camera heights, but do so with an understanding that a vertically level camera is going to keep vertical lines in the frame straight. Windows, corners, furniture and other items will provide those straight lines. Aperture Selection for Christmas Tree Photography
While a wide variety of apertures can be used for Christmas tree photos and blurred Christmas trees are quite beautiful, f/16 is one of my favorites. I know, your first thought was to cringe at the softness that diffraction will impart at this aperture and that is a true concern. But, the narrower the aperture used, the bigger the star effect created by each light on the tree. The f/16 aperture is a bit of a compromise in that the images remain reasonably sharp (and sharpenable) with rather large stars being created. Experiment with f/8 through f/22 to determine your own preference.
While ISO 100 is ideal, I went to ISO 200 for this exposure to reduce the amount of time each frame was taking. This one stop increase in ISO meant little in terms of noise, but it took 15 seconds off of the exposure and another 15 seconds off of the long exposure noise reduction information capture following the exposure. Summary
You have spent (or are going to spend) all that time and expense putting the Christmas tree up and decorating it, so ... plan on spending some time taking pictures of it. If you don't think the right lens is in your kit, this would be a good time to buy it. Renting a lens to use over the holidays is another great idea.