Wearing green and decorating with shamrocks are a given on March 17th, but the origins of these traditions goes back hundreds of years with Saint Patrick. Saint Patrick is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish.
The earliest evidence of this anecdote first appears in writing in 1726, though it may be older. The color green has been associated with Ireland since at least the 1640s and green ribbons and shamrocks have been worn on St. Patrick's Day since at least the 1680s. Throughout the 19th and 20th centuries, the association of the color green and with St Patrick's Day grew.
Here's something really interesting to me... in Buenos Aires, a huge party is held in the downtown street of Reconquista every St. Patrick’s Day, where the Irish community is the fifth largest in the world outside Ireland. Irish emigrants arrived in Argentina mostly between 1830 and 1930.
In fact, the Irish community in Argentina is the largest in any non-English speaking country in the world! The Irish-Argentines are still trying to keep the inherited traditions alive and are working to rescue older traditions that have been lost over time. It’s believed that there are over a million Irish descendants in modern-day Argentina. One of their most famous or infamous descendant of the Irish is Che Guevara!
Today we celebrate in many ways and the shamrock is still the most significant symbol of the holiday!
(info from Wikipedia, images from BHG)