St. Patrick’s Day is celebrated every year on March 17, the anniversary of his death in the fifth century. Saint Patrick was a patron saint of Ireland and its national apostle. He was born in Roman Britain but was kidnapped at 16 and brought to Ireland. He escaped but later returned to Ireland and was credited with bringing Christianity to people. Since around the 9th or 10th century people in Ireland have been observing Roman Catholic feast honoring him on March 17. 
More than a century later homesick Irish soldiers serving in the English military march in the New York City on March 17 to honor the Irish patron saint. Liking for the St. Patrick’s Day parades in New York city, Chicago, and Boston and other American cities grew from there.
What do leprechauns have to do with St. Patrick’s Day? The icon of this holiday is a leprechaun, it’s a belief in leprechauns probably stems from Celtic belief in fairies, tiny men and women who could use their magical powers to serve good to evil powers. Many of people on this holiday dress up at these trickeries.
Why is the holiday celebrated in green? The color green symbolizes many things for them like their flag color and the shamrock as well as the islands nickname. Before it was celebrated in green many wore blue to celebrate the holiday.