Section 84. The city council shall fix suitable times for its regular meetings. The mayor, or any four members of the city council, or any three members thereof in the case of a city council composed of seven members, may at any time call a special meeting by causing written notices, stating the time of holding such meeting and signed by the person or persons calling the same, to be delivered in hand to each member of the city council, or left at his usual dwelling place, at least twelve hours before the time of such meeting. Meetings of the city council may also be held at any time when all the members of the city council are present and consent thereto. Except in the case of executive sessions authorized by section twenty-three A of chapter thirty-nine, all meetings of the city council shall be open to the press and to the public, and the rules of the city council shall provide that citizens and employees of the city shall have a reasonable opportunity to be heard at any such meeting in regard to any matter considered thereat.