Full Text of the 20th Amendment
Section 1. The terms of the President and 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.
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 elect 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.
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.
Section 5. Sections 1 and 2 shall take effect on the 15th day of October following the ratification of this article.
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.
This so-called "Lame-Duck" amendment reduced the previous four-month period between the November elections and the March 4 starting date of congressional and presidential terms. This ended the custom, when both terms expired on the same day, that required outgoing presidents to sit outside the Senate chamber waiting to sign last-minute legislation. Also, under this amendment, if a presidential election were thrown into the House of Representatives following a deadlock in the January 6 counting of electoral ballots, that decision would be made by a newly elected House rather than one set to go out of existence on March 4. Source: U.S. Senate
The four-year term of the president and vice president was fixed by the Constitution in Ariticle II, Section 1. Because time was needed for new members to settle their affairs at home before traveling to Washington to join Congress, March 4 was initially chosen as the date both a new president and Congress would take office. However, as transportation and communications improved, this meant that the departing Congress and president would remain in office for an unnecessarily long time following the November elections. By moving the beginning of the president’s new term from March 4 to January 20 (and in the case of Congress, to January 3), proponents of Amendment XX hoped to put an end to the “lame duck syndrome (where those who were not reelected had little power to push through their policies), while at the same time allowing for a speedier transition for the new administration and legislators. The amendment was ratified on January 23, 1933.
Amendment XX also provides for succession plans if the newly elected president or vice president is unable to assume his or her position. If the president is not able to hold office, the vice president will act as president. Amendment XX gives Congress the power to pass legislation outlining a more detailed succession plan if the vice president is also not able to carry out the presidential duties until a new president and vice president are qualified. Source: Annenberg Classroom