On which day of the week did the first Earth Day occur?

The first Earth Day occurred on Wednesday, April 22, 1970.

U.S. Senator Gaylord Nelson proposed a nationwide environmental teach-in after growing public concern about pollution, oil spills, pesticides, and industrial damage. Denis Hayes coordinated the event, organizing demonstrations and educational activities at thousands of sites across the United States.

The Wednesday date was chosen strategically: it fell on a weekday between many students’ spring breaks and final examinations, making campus participation more likely. This is why some accounts describe the date-selection logic, rather than merely treating April 22 as a fixed annual observance.

An estimated 20 million Americans participated. The event helped build political momentum for major environmental measures, including the creation of the Environmental Protection Agency later in 1970 and passage of the Clean Air Act amendments. Earth Day is now observed internationally every April 22.