According to government archives, the 20th Amendment to the U.S. Constitution was ratified on January 23, 1933. Section 1 has had the most direct impact and moved up the dates on which elected presidents and congressmen took office, with the president elect assuming office on January 20 following election day and elected congressmen taking office on January 3. The swearing-in of the president and vice president being moved from March to January was of significant importance because it reduced the "lame duck" period during which the presidential administration was considered relatively useless.
Reducing the "lame duck" period for both Congress and the presidency was an important bit of legislation and occurred because of the ongoing Great Depression. America needed help fast but had to wait a long time for the newly-elected Franklin D. Roosevelt to replace the defeat Herbet Hoover, creating a relative vacuum of power during which Roosevelt had not yet taken office but Hoover had little motivation to act. Essentially, a "lame duck" period can be harmful if a defeated incumbent decides to stop performing his or her duties to a reasonable degree, creating stagnation. Even worse, a "lame duck" president or congressman could intentionally throw wrenches in the path of the incoming replacement by flooding legislative or executive channels with worthless bills or questionable appointments, creating a political mess for the replacement to handle. Congress wisely decided that transfers of power should be concluded quickly.
Section 2 of the 20th Amendment asserts that Congress shall meet at least once a year, beginning on January 3. This is important because it forces Congress to be active. While this is not a problem today, with Congress being regularly in session, the dilemmas of inactive legislatures were rather prominent early in American history. After the Revolutionary War most Americans were farmers or had jobs linked to agriculture, making it difficult for them to travel routinely to Washington, D.C. to vote on legislation. This limited Congress to only the wealthiest citizens who could afford to leave their farms in the hands of trusted employees. Legislatures often met only infrequently and absenteeism would be common. Section 2 sets the expectation that the U.S. Congress is a professional, proactive legislature unlike lawmaking bodies of previous generations. It acknowledges the complexities of the day, especially the current Great Depression, and insists that duly elected congressmen will work hard to shape a successfully-run nation.
Sections 3 and 4 help answer difficult questions of presidential succession and assert that Congress shall determine who will be both president and vice president in the event of extenuating circumstances. The House will determine the president and the Senate will determine the vice president. This is important because of historical troubles related to executive succession: Many nations and kingdoms were thrown into chaos after a ruler perished unexpectedly and factions quickly fought for control. By establishing that Congress alone has the power to determine who shall be president and vice president in the event of a crisis the Constitution has helped eliminated the possibility of coup or internal struggle.