Thousands of people are expected to take to the streets in cities across America and across the world on Monday in May Day demonstrations.

Considered by many a celebration of spring days to some, others look at May 1 as a day to protest everything from worker’s rights to oppressive policies to immigration reform.

Here’s a look at the history of May Day and why some people choose it as a day of protest.

What is May Day?

May Day was first celebrated thousands of years ago as a spring festival that included dancing – in some places using ribbons and circling a "Maypole" – singing and eating cake. Floralia, or the festival of Flora the Roman goddess of flowers, was held in ancient Rome on April 27 and is believed to be the first May Day-type celebration on record.

The date is also linked to the Gaelic May Day festival of Beltane, which means “the return of the sun.”

If It’s about spring, then why are people marching in protest?

While May Day is marked as a spring celebration by many, it is also known as International Workers' Day, and is seen as a day to celebrate and promote advances for the working class. Many people have the day off from work in other countries, and take part in marches for workers' rights and for other social issues on May Day.

Why do they choose May 1 to protest?

In 1891, the Second International, a worldwide socialist and communist parties organization, chose May 1 as a day to march for workers' rights to commemorate a demonstration that had taken place in Chicago five years earlier. What became known as the Haymarket Affair began as a protest for an eight-hour workday that turned violent when a bomb went off killing police officers who were monitoring the gathering. After the bomb exploded, police began shooting into the crowd of protesters, killing four. The Second International declared May Day a date of remembrance for the protesters who were killed, blaming the police for attacking what they said was a peaceful protest.

Is the United States the only country that celebrates May Day with protests?

No. Countries across the globe mark May 1 as a day to celebrate labor and to highlight the problems workers face. Marches are planned across the world, and the day is a holiday in many countries.

What’s going on in the United States?

On Monday there will be rallies in Seattle, a city that has a history of May Day protests, in Minneapolis, in Miami and in at least 200 other U.S. cities in 41 states. Many of the protests, such as the one set for Washington D.C., will focus on immigration and the role of immigrants in the American workplace.

"There's a real galvanization of all the groups this year," Fernanda Durand of CASA in Action told USA Today. Durand will lead a march of about 10,000 people in Washington for immigrants' rights Monday. "Our presence in this country is being questioned by Donald Trump. We are tired of being demonized and scapegoated. We've had enough."

Other marchers will be protesting for LGBT rights and women’s pay issues.

It seems that many of the protests are aimed at President Donald Trump. What does he say about it?

Trump issued a proclamation on Friday declaring Monday "Loyalty Day," saying supporters should "recognize and reaffirm our allegiance to the principles" that make the country great. He's asking that government buildings prominently display the U.S. flag and asking schools to somehow observe "Loyalty Day."

The proclamation reads in part:

“The United States stands as the world's leader in upholding the ideals of freedom, equality, and justice. Together, and with these fundamental concepts enshrined in our Constitution, our Nation perseveres in the face of those who would seek to harm it," the proclamation reads.

"We humbly thank our brave service members and veterans who have worn our Nation's uniform from the American Revolution to the present day. Their unwavering loyalty and fidelity has made the world a safer, more free, and more just place. We are inspired by their pride in our country's principles, their devotion to our freedom, and their solemn pledge to protect and defend our Constitution against all enemies, foreign and domestic."

It is not the first time May Day has been called Loyalty Day. In 1921, the U.S. government declared May 1 “Loyalty Day.”