Why does a President’s term end on January 20?

"the Twentieth Amendment was designed to remove the excessively long period of time a defeated president or member of Congress would continue to serve after his or her failed bid for reelection"

https://www.britannica.com/topic/Twentieth-Amendment

president thomas jefferson hoped that the embargo act of 1807 would the united states by demonstrating to britain and france their dependence on american goods, convincing them to respect american neutrality and stop impressing american seamen. instead, the act had a devastating effect on american trade.