Saint Patrick's Day (The Festival of Patrick); is a cultural and religious holiday celebrated on March 17. It commemorates Saint Patrick (c. AD 387–461), the most commonly recognized of the patron saints of Ireland, and the arrival of Christianity in Ireland. 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 Irish have observed this day as a religious holiday for over 1,000 years.
The day is generally characterized by the attendance of church services, wearing of green attire, 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 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.
Some American cities, such as Chicago, have developed their own tradition -- the annual dyeing of the Chicago River green. The practice started in 1962, when city pollution-control workers used dyes to trace illegal sewage discharges and realized that the green dye might provide a unique way to celebrate the holiday. That year, they released 100 pounds of green vegetable dye into the river--enough to keep it green for a week! Today, in order to minimize environmental damage, only 40 pounds of dye are used, and the river turns green for only several hours.
Luck of the Irish refers to the abundance of good fortune.
Sources: Wikipedia; History.com