Saint Patrick's Day was made an official feast day in the early seventeenth century, and has gradually become a celebration of Irish culture in general. The day is generally characterized by the attendance of church services, wearing of green attire, public parades and processions, and the lifting of Lenten restrictions on eating, and drinking alcohol, which is often proscribed during the rest of the season.
Saint Patrick's Day is a public holiday in the Republic of Ireland, Northern Ireland, Newfoundland and Labrador and in Montserrat. It is also widely celebrated by the Irish diaspora, especially in places such as Great Britain, Canada, the United States, Argentina, Australia, and New Zealand, among others. Today, St. Patrick's Day is probably the most widely celebrated saint's day in the world.
Originally, the color associated with Saint Patrick was blue. Over the years the color green and its association with Saint Patrick's day grew. Green ribbons and shamrocks were worn in celebration of St Patrick's Day as early as the 17th century. Saint Patrick is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish, and the wearing and display of shamrocks and shamrock-inspired designs have become a ubiquitous feature of the day.
Okay, enough of this blathering! Here is the answer...
(Information about St. Paddy's Day was collected from wikipedia.com and history.com.)
As you can tell, this is a trick question. Although, people have been 'observing' the feast day as a religious holiday for many years, they did not actually 'celebrate' it until the 18th century. :P