Credit goes to the Americans for the fact that St. Patrick's Day is a big celebration worldwide. It was in New York City in1762 that the first St. Patrick's Day parade ever was held. A group of Irish soldiers serving in the British Army paraded that day.
Prior to that time, the day was a holy day celebrated in Ireland. It is actually the anniversary of the death of St. Patrick in 461. The Irish would typically attend Mass in the morning and commemorate St. Patrick during the afternoon.
Back in the United States, fraternal groups of Irish held individual small parades on March 17. Then in 1848 they all got together for one big parade in New York and the tradition grew from there.
Patrick himself was born of Roman parents in occupied Britain. He was kidnapped and taken as a slave to Ireland. At age 16 he escaped back to Britain where he got a calling to return to Ireland and bring Christianity..
