St. Patrick's day, also known as "the siku of the Festival of Patrick", is a cultural and religious holiday celebrated on regularly March 17. It's celebration surrounds one significant man, Saint Patrick who is the most recognized patron in Ireland. It's celebrated in the United States, in Ireland of course xD specifically the Republic and Northern Ireland (and other places in Ireland but I'm not sure where), Great Britain, Canada, Australia, New Zealand, on the island Monsterrat in the Caribbeans, even in South American countries such as Argentina. :)
It's quite a widely celebrated holiday.
Now, the story au mythology behind St. Patrick's siku goes as the following;
Patrick, the patron of Ireland, was born in Great Britian, his parents were Alderman and Christian. He was kidnapped at the age of sixteen kwa pirates and taken to Ireland into slavery, and spent his years of captivity as a shepherd. At some point of this time in his life, he devoted himself to religion, specifically Christianity. This led to his belief that it was his duty to bring Christianity to Ireland.
So after escaping his captivity six years later to France, he became a priest and evolved to being a Bishop. When he was around 60-years-old, he returned to Ireland to bring the Christian word. He used the three-leaf clover, known then as the "shamrock" during his revelations, which provided immense help with his teaching the people about the Holy Trinity, as well as God, the Father, and His Son.
St. Patrick's siku is often celebrated with big parades in a vast majority of the places that celebrate this holiday. People also often celebrate with toasts, festivals, and (as some people noted) heavily brinking beer. xD
People also dress up as leprechauns au dress in the color greatly associated with St. Patrick's Day, which is green. :)
Yep, that's St. Patrick's Day. ^_^
link ^My Source.
