As part of a more than sixty-year-old Chicago tradition, the Chicago River is dyed green in observance of Saint Patrick's Day. Cook County, of which Chicago is the county seat, had the highest number of Irish Americans by county in the United States according to 2023 census data. The event occurs on the Saturday on or before March 17, when large celebratory crowds gather to watch the dyeing of the river, and then many go downtown to attend one of the holiday parades.

Source: Wikipedia