Saint Patrick's Day is celebrated each year on March 17th. In Ireland, Saint Patrick's Day is both a holy day and a national holiday.
Saint Patrick is the patron saint of Ireland as he was the one who brought Christianity to the Irish.
Saint Patrick told the people that the shamrock was like the idea of the Trinity, that in the one God there are three divine beings: the Father, the Son and the Holy Spirit.
The shamrock was sacred to the Druids, so Saint Patrick's use of it in explaining the trinity was very wise.