Amendment XX:

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.

in this picture you see a headline of President Kennedy's assassination being announced and Vice President Johnson being sworn in this is a good example of the vice president having to step up.

Section 1: Explains that the president ends January 20th and the new president elect enters into what we know as the white house and congress moves out January 3rd
Section 2:
The beginning begins January 3rd for Congress unless stated otherwise.
Section 3:
If the president cannot take on the role next in line is the vice president and if the vice president fails to step up congress then chooses someone to act in that position.
Section 4:
Explains when the congress gets to choose the president and how
Section 5:
self-explanatory.

picture of President Obama being sworn in the White House on January 20th when first became president