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 Chritianity to the Irish.
According to legend, Saint Patrick used the shamrock, which looks like clover, as it has three leaves on each stem. Saint Patrick told the people that the shamrock was the trinity - that in the one God there were trhee devine 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.