Everyone’s Irish on St. Patrick’s Day, and that means everyone — worldwide.

Somehow a local celebration of a revered holy man became a global celebration.

St. Patrick himself is a sympathetic figure. Most of what is known about the 5th century missionary and bishop comes from The Confessio — written by the man himself. In it, Patrick, who is thought to have been born in Roman Britain, tells of being kidnapped at 14 by Irish raiders who enslaved him. Patrick wrote that, after six years as a slave, the voice of God guided him to a ship waiting to take him home. In Britain, he studied to become a priest before returning to Ireland. There he worked miracles and converted thousands to Christianity, famously using a shamrock to explain the Holy Trinity.

Patrick is said to have died in 461 on the 17th of March, a day that became a Christian feast day in the early 17th century.

It started as a public holiday in a few places – the Republic of Ireland, Northern Ireland, a British colony called Montserrat and two Canadian provinces, Labrador and Newfoundland.

What started small is now a global celebration. In the 21st century, there are celebrations as far afield as New Zealand, Argentina, and Tokyo as well as closer to home in places like New Orleans in the US state of Louisiana and in the Canadian city of Montreal.

Experts say that while this has been partly influenced by the migration of the Irish around the world, the diaspora effect does not provide a full explanation. The festival also exists in places where there aren’t many ethnic Irish people and is celebrated in more countries than any other national festival.

Although criticized in many places for being commercial and promoting stereotypes of Irish culture, St. Patrick’s Day festivals around the world have very little in common apart from the thematic color green.

According to Patrick Griffin, a history professor at Notre Dame, there is nothing really Irish about the celebrations now with each location lending its own history to the event to create something unique. In Tokyo, for instance, where the festival began in 1992 and is largely organized by people who aren’t Irish, festivities focus on cultural exchange and unity and are managed by a non-profit called Irish Network Japan.

In Montreal, St. Patrick Day parades began when Irish soldiers in the British army observed the day while there on conquest. Its perpetuation has mainly been due to shared faith in Catholicism, say experts, rather than any desire to maintain a sense of Irish identity.