Today is St. Patrick's Day, the feast day of the patron saint of Ireland. There will be parades and celebrations in cities all across the world, but the holiday has always been most popular in the United States, especially in cities with large Irish-American populations. In Chicago, they dye the Chicago River green every year. And in New York City, there's a huge parade that goes up Fifth Avenue from 44th Street to 86th Street, past St. Patrick's Cathedral. And it's all to celebrate the man who brought Christianity to Ireland.