Saint Patrick's day, also know as the Feast of Saint Patrick, an Irish, religious celebration held every year on the 17th of March. Celebrations consist of colourful parades and parties. Irish people all over the world like to wear green coloured clothes and Shamrock on the day as an act of patriotism, if you find Shamrock with four leaves it's deemed to bring good luck.
For Catholics who abstain from alcohol, sweets etc... for lent, they are permitted to break the pledge on St Patrick's day. Its a public holiday in Ireland, many countries worldwide also like to celebrate the day.
Saint Patrick is the patron saint of Ireland, he was a fifth century British Christian missionary bishop.
It's widely believed that St Patrick was a missionary in Ireland during the second half of the fifth century, he banished snakes from the Island and used the three leaved Shamrock to teach the Holy Trinity to Irish pagans.