Not sure what you mean by assessment date? Lien date is January 1, 2018 but Cook County pays RE taxes in arrears. So in 2018 you'll be paying 2017 taxes. The notice of value date and appeal deadline dates vary depending on township. Revaluation occurs on triennial bases.
If you're referring to re- assessment dates, it varies per township. You can always check with the assessors website or with a property tax attorney.
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