I don't think we know a lot about ole St Patrick except he was a Christian missionary....In Ireland, me thinks...
and so the Irish celebrate St Patrick's Day, to honor him and their own culture.
A lot of Irish came to the USA and in reality helped found the country with their skills and culture.
So, every year we celebrate St Patrick's Day ...... kind of like us celebrating St Valentine's Day.
There's big celebrations, parades, people wearing green......green, green, green.