Earth Day

Earth Day is an annual event, celebrated on April 22nd, on which day events worldwide are held to demonstrate support for environmental protection. It was first celebrated in 1970, and is now coordinated globally by the Earth Day Network, and celebrated in more than 192 countries each year. 
The introduction of the name "Earth Day" was also claimed by John McConnell.
Nelson determined the week of April 19-25 was the best bet as it did not fall during exams or spring breaks. More students were likely to be in class, and there would be less competition with other mid-week events so he chose Wednesday, April 22.