The cabinet decides when the Diet is to be convened, and it is officially called to session by the promulgation of an imperial rescript. A Diet session may be designated as ordinary, extraordinary, or special. All Diet sessions, regardless of their particular designation, are referred to by numbers, as in " The _ th session of the Diet ".
An ordinary session of the Diet must be convened once a year in January to consider the national budget and related bills for the following fiscal year.
When an ordinary session is not meeting, an extraordinary session may be convoked whenever considered necessary by the Cabinet, for example, to consider urgent matters or a supplementary budget or bills for countermeasures against a disaster. When a quarter or more of the total Members of either House request that an extraordinary session be convoked, the Cabinet must convene a meeting. An extraordinary session must be convoked after a general election has been held following the expiration of the term of office for the Members of the House of Representatives, or after a regular election for the Members of the House of Councillors has been held.
A special session of the Diet must be convened following a general election called after the dissolution of the House of Representatives. When the Diet convenes in special session, the cabinet resigns en masse, and a new prime minister is chosen from among the members of either house.
The term of an ordinary session of the Diet is 150 days. The term of an extraordinary session or a special session is determined by a vote of both houses. The term of an ordinary session can be extended once, while the term of an extraordinary session or a special session can be extended twice.
When the House of Representatives is dissolved, the House of Councillors is closed at the same time, and the functions of the Diet stop. In times of national emergency, however, the Cabinet may convoke an Emergency Session of the House of Councillors during the period between the dissolution of the House of Representatives and the convocation of a Special Session of the Diet following the general election of Members of the House of Representatives. Because the House of Councillors executes proxy for Diet functions at the Emergency Session, measures taken are provisional and become null and void unless confirmed by the House of Representatives within a period of 10 days after the opening of the next session of the Diet. Such Emergency Sessions have been convoked only two times in the past.
At the beginning of each session, an Opening Ceremony attended by the Members of both Houses is held in the Chamber of the House of Councillors in the presence of His Majesty the Emperor. The Speaker of the House of Representatives gives a ceremonial address on behalf of the Members of both Houses and then the Emperor delivers a speech. The Opening Ceremony is held several days after Convocation Day.
Following the opening ceremony, several state ministers deliver speeches before a plenary sitting of each house. In an ordinary session of the Diet, the prime minister makes an address on general policy, the Minister for Foreign Affairs speaks on foreign policy, the Minister of Finance discusses financial matters, and the Minister of State for Economic and Fiscal Policy outlines economic policy. In an extraordinary session or a special session, the prime minister makes a speech; whenever necessary, other ministers also speak. Following the speeches, members are permitted to ask questions on behalf of their parties.