People in the U.S. immigrate from many countries. They bring their holidays with them. Indian-Americans often celebrate Diwali, Muslim-Americans often celebrate Eid.
St. Patrick’s Day originated as an Irish Celebration of an early missionary (someone who tries to convert others to his/her religion, in this case, Christianity). The myth that he “drove the snakes out of Ireland” referred to his conversion of the pagans (non-Christians) to Christianity. The shamrock (or clover) became associated with him because he used it to teach the trinity, the Christian idea that God was three-in-one: Father, Son, and Holy Spirit.
Over time, the holiday moved away from its religious roots and became associated with Irish pride. Ireland is a very green country, so people started wearing green on St. Patrick’s Day. (In many schools, a child not wearing green on St. Patrick’s day will be pinched by his friends.) The four-leaf-clover replaced the three-leafed-clover and became a symbol of luck rather than the trinity. There are often parades in cities with big Irish populations.
If you are ever in the U.S. on St. Patrick’s Day, look for a parade and go to a pub to try some green beer. Also, don’t forget to wear green so you can avoid getting pinched!
Erin Go Bragh!!
(This comes from an Irish language phrase, Éirinn go Brách, which expresses loyalty to Ireland.)
If you want to read more, here is a site with more information: Why Do We Celebrate St. Patrick’s Day? Fun St. Patrick’s Day Facts