Since the 1700's when the Catholic Church officially recognized the Feast of Saint Patrick on March 17, Saint Patrick's Day has been honored by the Irish and those of Irish descent around the world in celebration of their heritage. The shamrock, the leprechaun, and the color green are almost universally known the world over as symbols of Saint Patrick's Day, and by extension, Ireland in general.
Each year around the world parades and celebrations are held in major cities on Saint Patrick's Day, especially in Ireland, Britain, and North America. Don't be surprised if a local landmark turns green in celebration, either! Throughout the years every building from the Leaning Tower of Pisa to the Sydney Opera House have been lit up with emerald lights on the night of Saint Patrick's day. Niagara Falls is often lit up with green spotlights, and the Chicago River is annually dyed green for the day.
In recent years, Saint Patrick's Day celebrations have even moved out of his world to the International Space Station, where astronauts played Irish music and dressed in green to celebrate the occasion!