10 ILCS 5/25-4
In case of vacancies in the offices of Governor and Lieutenant-Governor, the officer performing the duties of the office of Governor, or if there is no such officer, the Secretary of State, shall issue a proclamation appointing a day for a special election to fill such vacancies, and shall issue a writ of election to the county clerks of the several counties in the state, and shall also, when necessary, call a special session of the General Assembly to canvass the votes cast at such election; but if such vacancy shall occur not more than ninety (90) days before a general election for members of the legislature, the vacancies shall be filled at such general election, in which case no special session of the General Assembly to canvass the votes shall be deemed necessary.
10 ILCS 5/25-4