No, it's not about drinking green beer and cooking out. Besides, who
grills Irish stew and cabbage anyway?
Take it from someone named Shawn -- It's a time for those of Irish descent to celebrate their ancestry. It started in Boston in 1737 by Irish immigrants as a Catholic feast day to celebrate/observe the death of St. Patrick who died on March 17, 461 AD. It became a day to show strength and unity among the Irish immigrants in the northeast after the potato famine hit and many were discriminated against in regards to employment and housing. They banded together and marched through the streets -- a tradition which later became the parades we see today.
If March 17 falls on a Sunday, the Catholic church calendars will mark the observation day as the Monday following 3/17. And if it falls during a Holy Week, they move it to the Monday following that week.
It also happens to be one of the few (if only) days that a Catholic will/might break Lenten abstinence or fasting and eat meat.