Final answer:
President Calvin Coolidge was the first US President to light up an outdoor national Christmas tree with electric lights in 1923.
Step-by-step explanation:
The first US President to light up an outdoor national Christmas tree with electric lights was President Calvin Coolidge in 1923. This event took place on Christmas Eve on the Ellipse, a park located south of the White House in Washington, D.C. The Christmas tree was adorned with 2,500 electric bulbs and attracted a large crowd.