Maybe you're going to plant a tree on Saturday. Or pick up some trash. (Litter, we're talking about.) You might be marching with scientists or maybe just spending some quality time with Mother Earth.

But however you spend it, Saturday is Earth Day – which got us wondering "How and why did Earth Day become a thing?"

We looked into it and here's a thumbnail version of what we found.

Roots

The first Earth Day observance happened in 1970. Understanding where it came from means looking a little further back, though. In particular, three things from the 1960s are worth mentioning.

Silent Spring by Rachel Carson

This best-selling book came out in 1962 and introduced a lot of people to the connections between industry and the environment. Carson wrote about damage to wildlife from pesticides and other chemicals, especially the effects of DDT on birds. She raised the prospect that – without some changes – the spring migrations of some birds could become memories instead of annual rituals.

Demonstrations, marches, social change

As the decade of the '60s wore on, more people were joining rallies, marches, or protests to try to bring about change. Young people on college campuses were especially important to the civil rights and anti-war movements. As those movements produced some changes in policy, their methods got picked up by advocates for women's rights, gay rights, and the environment.

Big oil spill of 1969

An accident at an oil drilling platform off the California coast near Santa Barbara led to about 3 million gallons of crude oil spilling into the Pacific Ocean in 1969. The oil slick was about 35 miles long, the Los Angeles Times says, and it killed thousands of birds, fish, and sea mammals.

These days, oil spills sometimes lead to criminal prosecution for breaking environmental laws. Back in 1969 there weren't really an environmental laws to break. That would soon change, though.

Sen. Gaylord Nelson comes up with the idea

Pretty much every account of Earth Day's origins gives Sen. Gaylord Nelson of Wisconsin the credit for coming up with the idea.

The Earth Day Network says Nelson described the concept as "a national teach-in on the environment."

To make it more effective he did something that's pretty rare nowadays. Nelson, a Democrat, recruited one of his Republican colleagues who was also concerned about the environment, Sen. Pete McCloskey, to co-chair the event. To tap into the activism of young people they looked to a university campus – Harvard – to find their national coordinator, Denis Hayes, who put together events around the country.

The first Earth Day was on April 22, 1970, and an estimated 20 million Americans took part.

Later that year the Environmental Protection Agency was created and soon after that the Clean Air Act, the Clean Water Act, and the Endangered Species Act all became laws.

Over the years Earth Day went global and the number of people involved swelled. The Earth Day Network expects about 1 billion people will take part in activities on Saturday.