There are three kinds of Diet sessions: ordinary, extraordinary, and special. The Emperor convokes each session with the advice and approval of the Cabinet.An ordinary session of the Diet is usually convoked once a year in January with a term of 150 days. The Cabinet determines whether to convoke an extraordinary session. However, when a quarter or more of all the members of either house so demand, the Cabinet must convoke an extraordinary session. It is usual for one or two extraordinary sessions to be convoked from summer to autumn every year. In addition, an extraordinary session must be convoked after a general election caused by the expiration of the term of office of the members of the House of Representatives and after an ordinary election of the members of the House of Councillors. A special session must be convoked within 30 days of a general election of the members of the House of Representatives occasioned by dissolution. In a special session, the prime minister is chosen from among the members of the Diet; the Cabinet must resign en bloc upon the convocation of a special session.
The term of an extraordinary session or of a special session is determined by a concurrent vote of both houses. The term of a session may be extended when both houses agree by a concurrent vote. If the two houses do not come to agreement on these matters, or the House of Councillors fails to come to a decision, the decision of the House of Representatives prevails.
During a session, a plenary gathering of the House of Representatives usually meets at one o'clock in the afternoon on Tuesdays, Thursdays, and Fridays, and a plenary gathering of the House of Councillors usually meets at ten o'clock in the morning on Mondays, Wednesdays, and Fridays. The speaker of the House of Representatives or the president of the House of Councillors has the power to call a plenary gathering on any other day when he or she deems it necessary.