St. Patrick’s Day honors Ireland’s patron saint, St. Patrick, as an official Christian religious holiday and now doubles as Ireland’s national day. Green, a color from Ireland’s flag, is central: Irish people and descendants globally wear it to mark the occasion. The holiday thrives in northern U.S. states, where Irish heritage is stronger, unlike the South with its larger African American population and fewer Irish descendants.

The river-dyeing ritual, iconic in Chicago, uses a harmless vegetable-based dye, turning the water bright green for hours. This vivid display, paired with parades and green attire, embodies the joyous, cultural pride of the celebration.