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,

St. Patrick's Day is Monday, March 17, and chains such as Krispy Kreme and Burger King are offering deals and discounts to celebrate the holiday.

If it's March, and it's green, it must be St. Patrick's Day. The day honoring the patron saint of Ireland is a global celebration of Irish heritage. And nowhere is that more so than in the United States,

It was among Irish American communities that the day became the celebration it is, from its roots as a more solemn day with a religious observance in Ireland.