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The 20th Amendment, which has been nicknamed “The Lame Duck Amendment,” was ratified in January of 1933. Among other things, this amendment shortened the time that politicians in both the Congress and the Presidency could remain in office after an election. The inauguration day for the president was moved to January 20, while the start date for Congress was bumped up even further to January 4. This two-week difference between the inauguration of new members of Congress and the incoming president was established to give Congress time to respond to a situation in which the regular voting process was unable to determine a president-elect, the candidate died, or some other scenario where there was no president to be sworn in.
https://www.atlasobscura.com/articles/why-is-inauguration-day-on-january-20