St. Patrick’s Day usually conjures images of partying, Catholicism, Irish nationalism and, perhaps most famously, the color green: green clothes, green shamrocks, green beer and green rivers. But the holiday used to be a solemn feast day when you’d be far more likely to see the color blue.

Wearing green is one of the most important aspects of Irish culture and celebrating St. Patrick’s Day, but not everyone knows why people wear green when celebrating the holiday worldwide. According to TIME Magazine,

The Chicago River is once again glowing kelly green as the city opens its annual St. Patrick’s Day celebrations