
People usually celebrate this date with green items and shamrock because Ireland where St. Patrick spread Christianity, is sometimes called “The Emerald Isle” because a lot of the country is a beautiful green, like the color of emeralds and St. Patrick used a shamrock like clover to explain the Christian message of Trinity (Tri+Unity) simply and shamrock becomes a symbol of Christianity.
Also, people usually drink Guinness, which was originally made in Ireland.
You can see the Chicago River colored in green in Chicago and the tower lighted up in green in New Zealand. Also, you can see many big parades in the U.K, the U.S., Canada, Australia etc where many immigrants from Ireland live.