75 ILCS 10/14

In the event that the board of library directors or trustees of any public library or the governing authority of any other type of library determines to withdraw from a library system, the board of library directors or trustees or the governing authority shall submit notice to the library system of the intent to withdraw and serve a copy thereof upon the State Librarian. Any such notice shall be filed on or before April 1st of any year, and shall be effective on or before June 30th of the next ensuing year.