35 ILCS 200/21-145
At the same time the county collector annually publishes the collector's annual sale advertisement under Sections 21-110, 21-115, and 21-120, counties may, if the county board so orders by resolution, publish an advertisement giving notice of the intended sale of certain tax liens and certificates that have been forfeited and are held by the county pursuant to Section 21-90 . Under no circumstance may a tax year be offered at a scavenger sale prior to the annual tax sale for that tax year (or, for omitted assessments issued pursuant to Section 9-260, the annual tax sale for that omitted assessment's warrant year, as defined herein).
The county collector shall include in the advertisement and in the application for judgment and sale under this Section and Section 21-260 the total amount of all general taxes upon those properties which are delinquent as of the date of the advertisement. In lieu of a single annual advertisement and application for judgment and sale under this Section and Section 21-260, the county collector may, from time to time, beginning on the date of the publication of the annual sale advertisement and before August 1 of the next year, publish separate advertisements and make separate applications on eligible properties described in one or more volumes of the delinquent list. The separate advertisements and applications shall, in the aggregate, include all the properties which otherwise would have been included in the single annual advertisement and application for judgment and sale under this Section. Upon the written request of the taxing district which levied the same, the county collector may also include in the advertisement the special taxes and special assessments, together with interest, penalties and costs thereon upon those properties which are delinquent as of the date of the advertisement. The advertisement and application for judgment and sale shall be in the manner prescribed by this Code relating to the annual advertisement and application for judgment and sale of delinquent properties.
As used in this Section, the term delinquent also includes tax liens and certificates forfeited to the county as trustee and held pursuant to Section 21-90, if those tax liens or certificates are approved for sale by the county board. Any tax lien or certificate held by the county pursuant to Section 21-90 that is offered at a scavenger sale shall be assigned by the county to the winning bidder at the scavenger sale as set forth in Section 21-90. After 4 years from the date of assignment, the assignment is void and the tax certificate shall be forfeited back to the county and held pursuant to Section 21-90, unless a tax deed has been issued and recorded by the assignee or a court order to toll the deadline pursuant to Section 21-90 is entered.
As used in this Section, "warrant year" means the year preceding the calendar year in which the omitted assessment first became due and payable.
35 ILCS 200/21-145