Perhaps you’ve heard that St. Patrick is remembered for driving the snakes out of Ireland, as pictured here. This is actually a euphemism for driving out pagan worship and converting Ireland to Christianity. Within a few centuries, St. Patrick had become the patron saint of Ireland, and his life has been celebrated on March 17th for centuries around the world. Pinching people that don’t wear green? That tradition was started by school children.
Read more at Widipedia.