The Constitution of the United States had established March 4 as its Inauguration Day in order to allow enough time after Election Day for officials to gather election returns and for elected candidates to travel to the capital.
That transition period is no longer necessary thanks to advancements in the voting system and, of course, ease of transportation. The passage of the 20th Amendment officially moved the date to Jan. 20.
Around noon on that day, the president-elect will recite the 35-word oath of office and will be sworn in by the Chief Justice of the United States Supreme Court.
– The Arizona 100 reports