It is a celebration of Irish history and culture, and is a national holiday in the Republic of Ireland and Northern Ireland.
Saint Patrick is the Patron Saint of Ireland and is celebrated for bringing Christianity to the country.
He is thought to have grown up in Britain, during Roman times, but was kidnapped and taken to Ireland as a slave.
Once he was free again, he became a priest, and went back to Ireland to convert thousands of people to Christianity.
Saint Patrick's Day started as a religious feast to celebrate Saint Patrick's work, but it's grown to be an international festival of all things Irish.
People take part in parades and dancing, eat Irish food, and enjoy firework displays.
The day is also famous for people wearing shamrocks, dressing up as bearded Irish fairies called leprechauns, and wearing all green.
Millions of people around the world celebrate Saint Patrick's Day on the 17th of March, every year.