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.

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.

The First Parade
The first St. Patrick’s Day parade took place not in Ireland but in the United States.

Irish soldiers serving in the English military marched through New York City on March 17, 1762. Along with their music, the parade helped the soldiers reconnect with their Irish roots, as well as fellow Irishmen serving in the English army.

Over the next 35 years, Irish patriotism among American immigrants flourished, prompting the rise of so-called "Irish Aid” societies like the Friendly Sons of Saint Patrick and the Hibernian Society.

Each group would hold annual parades featuring bagpipes (which actually first became popular in the Scottish and British armies) and drums.

In 1848, several New York Irish Aid societies decided to unite their parades to form one New York City St. Patrick’s Day Parade. Today, that parade is the world ‘s oldest civilian parade and the largest in the United States, with over 150,000 participants.

Each year, nearly three million people line the 1.5-mile parade route to watch the procession, which takes more than five hours. Boston, Chicago, Philadelphia and Savannah also celebrate the day with parades involving between 10,000 and 20,000 participants.

No Irish Need Apply
Up until the mid-19th century, most Irish immigrants in America were members of the Protestant middle class.

When the Great Potato Famine hit Ireland in 1845, close to a million poor and uneducated Irish Catholics began pouring into America to escape starvation.

Despised for their religious beliefs and funny accents by the American Protestant majority, the immigrants had trouble finding even menial jobs.

When Irish Americans in the country’s cities took to the streets on St. Patrick’s Day to celebrate their heritage, newspapers portrayed them in cartoons as drunk, violent monkeys.

However, the Irish soon began to realize that their great numbers endowed them with a political power that had yet to be exploited.

They started to organize, and their voting block, known as the "green machine,” became an important swing vote for political hopefuls.

Suddenly, annual St. Patrick’s Day parades became a show of strength for Irish Americans, as well as a must-attend event for a slew of political candidates.

In 1948, President Truman attended New York City ‘s St. Patrick’s Day parade, a proud moment for the many Irish whose ancestors had to fight stereotypes and racial prejudice to find acceptance in America.

Wearing of the Green Goes Global
Today, St. Patrick’s Day is celebrated by people of all backgrounds in the United States, Canada and Australia.

Although North America is home to the largest productions, St. Patrick’s Day has been celebrated in other locations far from Ireland, including Japan, Singapore and Russia.
In modern-day Ireland, St. Patrick’s Day has traditionally been a religious occasion.

In fact, up until the 1970s, Irish laws mandated that pubs be closed on March 17.

Beginning in 1995, however, the Irish government began a national campaign to use St. Patrick’s Day as an opportunity to drive tourism and showcase Ireland to the rest of the world.

Last year, close to one million people took part in Ireland ‘s St. Patrick’s Festival in Dublin, a multi-day celebration featuring parades, concerts, outdoor theater productions and fireworks shows.

The Chicago River
Chicago is famous for a somewhat peculiar annual event: dyeing the Chicago River green. The tradition 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, making the river green for only several hours.

Although Chicago historians claim their city’s idea for a river of green was original, some Savannah natives believe the idea originated in their town.

They point out that, in 1961, Savannah mayor Tom Woolley had plans for a green river. Due to rough waters on March 17, the experiment failed, and Savannah never attempted to dye its river again.

History.com