Which month did the U.S. President and Vice President begin their terms in office before the 20th Amendment of the U.S. Constitution was ratified?

U.S. Presidential terms began March 4th, before the 20th Amendment was ratified. The precedent for March 4th was set after the new Constitution was ratified. The Congress of the Confederation (the governing body of the United States of America prior to the U.S. Constitution) set that date for starting the newly reorganized government. The Constitution did not specify any specific day for the start of the term. By the time of the 2nd presidential election in 1792, Congress established by law that presidential electors are to be chosen during November or early December.

The result of the term starting nearly 4 months after the election was a long "lame duck" period when little work tended to be accomplished. It also meant that a lame duck Congress and presidential administration could fail to adequately respond to a significant national crisis in a timely manner during that time between November and March.

Two presidents, Abraham Lincoln in 1861 and Franklin Roosevelt in 1933 each had to wait the four months so they and the incoming Congress could deal with secession of the Southern states (Lincoln) and the Great Depression (Roosevelt).

The U.S. Congress proposed the 20th Amendment March 2, 1932, by January 1933, Missouri was the 36th state to ratify it, allowing it to be added as an amendment of the Constitution. The first presidential and vice presidential terms to begin under were the second terms of Franklin Roosevelt and John Nance Garner, on January 20, 1937.