The temperature of the water for a Christmas tree should be cool but not too cold. A good rule of thumb is to use room temperature water, as this will prevent the tree from losing too much water and becoming dehydrated. It's also important to keep the tree away from any sources of heat, as this can dry it out and cause the needles to fall off. If you have a real Christmas tree, it's a good idea to check the water level daily and make sure it's still at a comfortable temperature.