Blue was the original colour associated with Saint Patrick (top patron saint of Ireland) until the 1798 rebellion when, on March 17th, soldiers wore full green uniforms to catch public attention and make a political statement. However, green ribbons and shamrocks were worn in celebration of St. Patrick's Day as early as the 17th century. The shamrock is thought to have been used by the saint to explain the Holy Trinity to the pagan Irish.
St. Patrick's Day is a public holiday in the Republic of Ireland, Northern Ireland, Newfoundland and Labrador and Montserrat. It's also celebrated by the Irish diaspora around the world, especially in Britain, Canada, US, Argentina, Australia and New Zealand. St. Patrick's Day once marked the arrival of Christianity to Ireland, but today it celebrates Irishness the world over. We noticed the people of Buffalo were ready for the festivities when we visited 2 weekends ago. Many homes and bars were already decorated for the occasion, prepared to party. I hadn't thought about St. Patrick's day in a long time, the Buffalo mania inspired me to pull this green outfit (I don't think I'll follow through on the day.)
Here's a local fact for sports fans: The Toronto Maple Leafs were known as the Toronto St Patricks from 1919-1927 and wore green jerseys. In 1999, they played on St. Patrick's Day and sported the retro uniforms.