St Patrick's Day is celebrated on March 17th and is the most important Irish festival. It commemorates St Patrick, the patron saint of Ireland, who brought Christianity to Ireland in the 5th century. While St Patrick's Day is a religious holiday in Ireland, in the United States it is celebrated with large parades, especially in New York City and Chicago where the river is dyed green for the occasion. Green is a symbol of Ireland and luck, and is widely worn on St Patrick's Day.