Prior to the enactment of the 22nd Amendment, ratified in 1933, a newly elected president was not sworn in until the March following the election. The 22nd amendment moved Inauguration Day to January 20 at noon. The 22nd amendment reads –

Section 1. The terms of the President and the Vice President shall end at noon on the 20th day of January, and the terms of Senators and Representatives at noon on the 3d day of January, of the years in which such terms would have ended if this article had not been ratified; and the terms of their successors shall then begin.

Section 2. The Congress shall assemble at least once in every year, and such meeting shall begin at noon on the 3d day of January, unless they shall by law appoint a different day….

Article II, Section 1, Clause 8, the Oath of Office clause states –

Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation: — “I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.” (Emphasis added)

The Chief Justice of the United States Supreme Court has historically administered the oath of office, although this is not required by the Constitution. Chief Justice John Roberts will administer the oath to president-elect Trump tomorrow afternoon.