Officers riding horses march during St. Patrick's day Parade on the Fifth Ave. in New York, the United States, March 17, 2011. The Fifth Avenue of New York City witnessed a sea of green on Thursday as the 250th annual St. Patrick's Day Parade attracted spectators from around the globe. St. Patrick's Day is a religious holiday celebrated internationally on March 17. It is named after Saint Patrick, the most commonly recognized of the patron saints of Ireland. [Xinhua/WuKaixiang]