Congressional terms begin and end on the same day, Jan. 3, every two years. This date was set by the Twentieth Amendment to help prevent candidates who have already been replaced in the elections from being in Congress for several months after voting has taken place.
Franklin Delano Roosevelt ratified the Twentieth Amendment on Jan. 23, 1933, which altered the original date of term changes from March 4 to the current date, setting terms to always change on odd years. The third day of January is considered the start of a new session of Congress, and the transitions between sessions affect the passage and deliberation of bills. A bill that is still under deliberation carries over between sessions, though a bill whose vote does not pass by the end of a session is automatically killed by the end of a session.