St Patrick's day is held on March 17th.
It is the date that Saint Patrick died. Saint Patrick is the man who is said to have introduced the catholic religion to Ireland. He tried to explain "the trinity" (3 people in the one god) with a three leafed plant called a shamrock. Hence the shamrock has significance for the day. Originally the day was important only in Ireland but because of the large numbers of Irish people who emigrated abroad and the colorfulness of the celebration the tradition spread around the world.
In Ireland St. Patrick's day is celebrated with a parade of floats, bands and marchers in each towns main street. Unfortunately because of the time of the year it invariably rains on the day.