Which constitutional amendment moved the start of U.S. presidential terms to January 20?

The Twentieth Amendment moved the start of U.S. presidential terms to January 20.

Section 1 of the Twentieth Amendment states that the terms of the president and vice president begin and end at noon on January 20. The amendment was ratified in 1933 and first applied to presidential and vice-presidential terms beginning in 1937.

Before this change, regular presidential inaugurations were generally held on March 4. The long interval between election and inauguration became known as the lame-duck period, especially when outgoing officials remained in office for months after a successor had been chosen. Moving the date shortened that transition.

The amendment also moved the start of congressional terms to January 3. It is sometimes confused with the Twenty-second Amendment, which limits presidents to two elected terms, or the Twelfth Amendment, which revised the Electoral College process. Neither of those amendments established January 20 as the term-start date.