Saint Patrick's Day.

The History of the Holiday

St. Patrick's Day is celebrated on March 17, his religious feast day and the anniversary of his death in the fifth century. The Irish have observed this day as a religious holiday for over a thousand years.
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.

The Chicago River

The Chicago River is really green.

Chicago is also famous for a annual event: dyeing the Chicago River green. The tradition started in 1962. 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 forty pounds of dye are used, making the river green for only several hours.

The Shamrock

The shamrock is a symbol of Irish nationalism.

The shamrock, which was also called the "seamroy" by the Celts, was a sacred plant in ancient Ireland because it symbolized the rebirth of spring. By the seventeenth century, the shamrock had become a symbol of emerging Irish nationalism. As the English began to make laws against the use of the Irish language and the practice of Catholicism, many Irish began to wear the shamrock as a symbol of their pride in their heritage and their displeasure with English rule.