Final answer:
The 20th Amendment to the U.S. Constitution, ratified in 1933, set the presidential inauguration date as January 20th. It aimed to reduce the 'lame duck' period and improve the efficiency of government transition.
Step-by-step explanation:
The amendment that states the inauguration will be on January 20th is the 20th Amendment to the United States Constitution.The 20th Amendment, which was ratified in 1933, significantly changed the dates on which the terms of the President, Vice President, and members of Congress begin and end. Prior to this amendment, there was a lengthy "lame duck" period after the election until the March 4th inauguration. The 20th Amendment reduced this period by moving the presidential inauguration to January 20th and the start of the new Congressional session to January 3rd. This adjustment was aimed at decreasing the inefficiency and legislative stalemate during the transition time between administrations, by shortening the time an outgoing president and Congress remained in office after the election. Conclusion Thus, the 20th Amendment is responsible for setting the inauguration date for the President of the United States to January 20th, streamlining the transition process and reducing the duration of a "lame duck" session in government.