The Mayor shall call special meetings of Council whenever in his opinion the public business may require it, or at the express written request of any four members of Council.  Whenever a special meeting shall be called, a summons or notice in writing signed by the Mayor or the President of Council shall be served upon each member of Council either in person or by notice left at his place of residence, stating the date and hour of the meeting and the purpose for which such meeting is called, and no business shall be transacted thereat, except such as is stated in the notice.  (1978 Code Sec. 2-3.)