A more thorough explanation:

The Twentieth Amendment is a change to the United States Constitution that was ratified in 1933. It made important changes to the dates of presidential and congressional inaugurations.

Before the Twentieth Amendment, presidential inaugurations were held on March 4 and congressional sessions began on the same day. This meant that there was a long period of time between the election of a new president and their inauguration, which was called the "lame-duck" period. During this time, outgoing officials still held their positions and could make important decisions, even though they were no longer accountable to the voters.

The Twentieth Amendment changed the date of presidential inaugurations to January 20 and congressional sessions to January 3. This shortened the lame-duck period and ensured that newly elected officials could take office more quickly.

For example, if the Twentieth Amendment had not been passed, a president who was elected in November would have to wait until March to take office. During that time, the outgoing president would still be making decisions and setting policies, even though they were no longer accountable to the voters. This could create confusion and uncertainty about the direction of the country.

The Twentieth Amendment also eliminated the short session of Congress that occurred between the November elections and the March inauguration. This session was often chaotic, as outgoing members of Congress tried to push through last-minute legislation and newly elected members tried to make their voices heard. By moving the start of the congressional session to January 3, the Twentieth Amendment ensured that all members of Congress would be elected and ready to work together from the beginning of the session.