Each numbered Congress spans two calendar years and is divided into two sessions: the first session (odd-numbered year, e.g., ) and the second session (even-numbered year, e.g., ). Modern Congresses begin on January 3 of odd-numbered years and end on January 3 two years later, unless a law sets a different date. The House is elected every two years, while Senators serve six-year terms staggered so that roughly one-third of the Senate is up for election every two years.