St. Patrick's Day celebrates the arrival of Christianity in Ireland and honors St. Patrick, the patron saint who was born in the 4th century, enslaved for six years, and is credited with using the shamrock to explain the Holy Trinity. The day is marked by public parades, festivals, and the wearing of green, symbolizing Irish heritage. Key historical points include St. Patrick's death on March 17, 493 AD, and the first St. Patrick's parade in New York in 1762.