Passed by Congress: 2 March 1932
Ratified: 23 January 1933
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.
When the Constitution was written in the late 18th century, things moved much more slowly than they do in modern times. Sailing across the Atlantic Ocean took several weeks; overland travel through the interior of the American continent was even slower. Long-distance communication meant sending letters through a rudimentary postal system or sending a messenger to hand-deliver important news. Therefore, the Framers of the Constitution designed a political calendar that left plenty of leeway for painfully slow travel and communication. Federal elections would thus be held in November, and the winners would be sworn into office in March—some five months later.
Fast forward to the 1930s. Couriers have been replaced by the telegraph, telephone, and radio, making long-distance communication virtually instant. Horses and buggies have been replaced by railroads, automobiles, and airplanes. It no longer takes nearly half a year for the winners of elections to learn they have won and make their way to Washington, DC. And the long delay between elections and the inauguration of the winning candidates has become increasingly problematic, as the so-called "lame duck" politicians—those who haven't won a new term, but remain in office from November through March—have no political capital and thus find it almost impossible to get anything done.
In 1932, this flaw in the political calendar is made all the more acute because the nation faces a grave crisis. The long winter of 1932-33—most of it occurring after Franklin D. Roosevelt defeats Herbert Hoover to win the presidency, but before Roosevelt's inauguration—happens to be the worst winter of the entire Great Depression. The defeated Hoover remains in the White House but does virtually nothing to deal with the crisis; Roosevelt, not yet in office, is likewise helpless to take action.
Before Roosevelt even takes office (in March 1933), the states ratify the 20th Amendment, changing the nation's political calendar to prevent such a mess from happening again in the future. The amendment shortens the "lame-duck" period by two months by moving inauguration day, for both the president and the members of Congress, up from March to January.
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.
The amendment also moved up the beginning of each session of Congress to early January—the same day chosen as the new inauguration day for US Senators and Representatives.
Section 3. If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.
The Twentieth Amendment also sought to resolve some ambiguity in the Constitution regarding what to do in a hypothetical scenario in which the winner of a presidential election dies before his inauguration day, or in which no candidate wins the presidency prior to the beginning of the term. (No such scenario has ever occurred in real life, before or after the passage of the Twentieth Amendment.)
Section 4. The Congress may by law provide for the case of the death of any of the persons from whom the House of Representatives may choose a President whenever the right of choice shall have devolved upon them, and for the case of the death of any of the persons from whom the Senate may choose a Vice President whenever the right of choice shall have devolved upon them.
This is another obscure hypothetical that has never arisen as a problem in reality. But thanks to the Twentieth Amendment, Congress could do something about it if it did.
Section 5. Sections 1 and 2 shall take effect on the 15th day of October following the ratification of this article.
This one's pretty self-explanatory. The new political schedule created by the Twentieth Amendment took effect on 15 October 1933.
Section 6. This article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by the legislatures of three-fourths of the several States within seven years from the date of its submission.