St. Patrick's Day is celebrated on March 17, the saint's religious feast day and the anniversary of his death in the fifth century. Saint Patrick, who lived during the fifth century, was a patron saint and national apostle of Ireland. He was born a Roman Britain, he was kidnapped and brought to Ireland as a slave at the age of 16. He escaped later in life, but returned to Ireland and was credited with bringing Christianity to its people. The Irish have observed this day as a religious holiday for over 1,000 years. The mythology surrounding his life became ever more ingrained in the Irish culture: Perhaps the most well known legend is that he explained the Holy Trinity (Father, Son and Holy Spirit) using the three leaves of a native Irish clover, the shamrock.
On St. Patrick's Day, which falls during the Christian season of Lent, Irish families would traditionally attend church in the morning and celebrate in the afternoon. Lenten prohibitions against the consumption of meat were waived and people would dance, drink and feast--on the traditional meal of Irish bacon and cabbage.
http://www.history.com/topics/st-patricks-day
