Patrick, a Romano-British former slave turned bishop, spent decades evangelizing Ireland after escaping captivity there as a youth. His death, traditionally dated to this day, became the basis for the feast day that later evolved into a global celebration of Irish identity, from Dublin parades to green rivers in Chicago.
Patrick's own writings, the Confessio and Letter to Coroticus, are the only certain sources for his life, and neither mentions a date of death. The March 17 tradition emerged centuries later, built up through hagiography that added shamrocks, snake-banishing, and other legends with little historical basis.
What is more certain is his extraordinary personal story: kidnapped from Britain by Irish raiders as a teenager, enslaved for six years, then guided by visions to escape and eventually return as a missionary to the people who had enslaved him. His writing carries a raw, first-person urgency rare for the era.
The feast day he inspired outgrew religious observance entirely. By the 18th century Irish soldiers in the British Army marked it with shamrocks; by the 20th, New York and Boston parades dwarfed anything held in Ireland itself, and Chicago began dyeing its river green in 1962, a tradition that continues today.
Key people: Saint Patrick